Hello,
based on the ticket from steelrat2000 @github
https://github.com/kaloz/mwlwifi/issues/174
I'm having exact the same problem 
I can't say from which version on this probem exists (because i own this router just for a week) so for me it's present in all versions of Lede. Currently using LEDE Reboot SNAPSHOT r4438-f5f1d40.
It seems its not a mwlwifi problem. Maybe a hostapd ?!
Could someone help me to solve this ?
Short description:
Router: Linksys WRT3200ACM-EU
OS: LEDE Reboot SNAPSHOT r4438-f5f1d40
5Ghz Wifi drops client connection after about (every) 2h20m
EDIT: Same problems with 2,4Ghz Wifi
/etc/config/wireless
config wifi-device 'radio0'
option noscan '1'
option type 'mac80211'
option hwmode '11a'
option path 'soc/soc:pcie-controller/pci0000:00/0000:00:01.0/0000:01:00.0'
option log_level '1'
option country 'DE'
option txpower '20'
option htmode 'VHT80'
option channel '36'
config wifi-iface 'default_radio0'
option device 'radio0'
option network 'lan'
option mode 'ap'
option ssid 'CR4P_2_5G'
option encryption 'psk2+ccmp'
option key '12345678901234' # 14 digits
Last Log when it happens:
Jun 23 18:48:08 OPENWRT hostapd: wlan1: STA ac:63:be:4a:13:22 WPA: group key handshake completed (RSN)
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 WPA: EAPOL-Key timeout
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 WPA: WPA_PTK: sm->Disconnect
Jun 23 18:48:08 OPENWRT hostapd: wlan0: AP-STA-DISCONNECTED f0:f6:1c:38:ae:c8
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 WPA: event 3 notification
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 IEEE 802.1X: unauthorizing port
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 WPA: EAPOL-Key timeout
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 WPA: WPA_PTK: sm->Disconnect
Jun 23 18:48:08 OPENWRT hostapd: wlan0: AP-STA-DISCONNECTED 28:a0:2b:e4:36:78
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 WPA: event 3 notification
Jun 23 18:48:08 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 IEEE 802.1X: unauthorizing port
Jun 23 18:48:10 OPENWRT kernel: [ 3625.527554] ieee80211 phy0: staid 5 deleted
Jun 23 18:48:10 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 MLME: MLME- DEAUTHENTICATE.indication(f0:f6:1c:38:ae:c8, 2)
Jun 23 18:48:10 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 MLME: MLME- DELETEKEYS.request(f0:f6:1c:38:ae:c8)
Jun 23 18:48:10 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 MLME: MLME- DEAUTHENTICATE.indication(28:a0:2b:e4:36:78, 2)
Jun 23 18:48:10 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 MLME: MLME- DELETEKEYS.request(28:a0:2b:e4:36:78)
Jun 23 18:48:10 OPENWRT kernel: [ 3625.565734] ieee80211 phy0: staid 4 deleted
Jun 23 18:48:13 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 IEEE 802.11: deauthenticated due to local deauth request
Jun 23 18:48:13 OPENWRT hostapd: wlan0: STA 28:a0:2b:e4:36:78 IEEE 802.11: deauthenticated due to local deauth request
Jun 23 18:51:13 OPENWRT kernel: [ 3808.400641] ieee80211 phy1: staid 2 deleted
Jun 23 18:51:13 OPENWRT hostapd: wlan1: STA 7c:dd:90:3a:32:c9 IEEE 802.11: associated (aid 2)
Jun 23 18:51:14 OPENWRT hostapd: wlan1: AP-STA-CONNECTED 7c:dd:90:3a:32:c9
Jun 23 18:51:14 OPENWRT hostapd: wlan1: STA 7c:dd:90:3a:32:c9 WPA: pairwise key handshake completed (RSN)
Jun 23 18:51:18 OPENWRT hostapd: wlan1: STA 7c:dd:90:3a:32:c9 IEEE 802.11: authenticated
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPDISCOVER(br-lan) 7c:dd:90:3a:32:c9
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPOFFER(br-lan) 192.169.178.61 7c:dd:90:3a:32:c9
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPDISCOVER(br-lan) 7c:dd:90:3a:32:c9
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPOFFER(br-lan) 192.169.178.61 7c:dd:90:3a:32:c9
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPREQUEST(br-lan) 192.169.178.61 7c:dd:90:3a:32:c9
Jun 23 18:51:21 OPENWRT dnsmasq-dhcp[1814]: DHCPACK(br-lan) 192.169.178.61 7c:dd:90:3a:32:c9 VU-SZ
Jun 23 18:53:18 OPENWRT hostapd: wlan1: AP-STA-POLL-OK ac:63:be:4a:13:22
Jun 23 18:57:16 OPENWRT hostapd: wlan0: STA f0:f6:1c:38:ae:c8 IEEE 802.11: authentication OK (open system)
cat /sys/kernel/debug/ieee80211/phy0/mwlwifi/info:
driver name: mwlwifi
chip type: 88W8964
hw version: 7
driver version: 10.3.4.0-20170606
firmware version: 0x09030007
power table loaded from dts: no
firmware region code: 0x30
mac address: 60:38:e0:bd:94:ba
2g: disable
5g: enable
antenna: 4 4
irq number: 45
ap macid support: 0000ffff
sta macid support: 00010000
macid used: 00000001
radio: enable
iobase0: e0e00000
iobase1: e1080000
tx limit: 1024
rx limit: 16384