Support for RTL838x based managed switches

@hurricos Beat me up, but with your code changes it just seems to work again somehow :face_with_open_eyes_and_hand_over_mouth:. Not sure if realtek-poe -d is supposed to return the prompt, it didn't print anything new when I plugged in a second PoE client. This is with your code (on 22.03):

root@OpenWrt:~# ubus call poe info
{
	"firmware": "v17.1",
	"mcu": "ST Micro ST32F100 Microcontroller",
	"budget": 70.000000,
	"consumption": 6.200000,
	"ports": {
		"lan1": {
			"priority": 2,
			"mode": "Legacy",
			"status": "Delivering power",
			"consumption": 3.100000
		},
		"lan2": {
			"priority": 2,
			"mode": "Legacy",
			"status": "Delivering power",
			"consumption": 3.100000
		},
		"lan3": {
			"priority": 2,
			"mode": "Legacy",
			"status": "Searching"
		},
		"lan4": {
			"priority": 2,
			"mode": "PoE",
			"status": "Searching"
		}
	}
}
root@OpenWrt:~# realtek-poe -d
Failed to add object: Invalid argument
TX -> 0x20 0x01 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0x18
Get system info

Configuration:

root@OpenWrt:~# cat /etc/config/poe 
config global
	option budget	'70'

config port
	option enable	'1'
	option id	'1'
	option name	'lan1'
	option poe_plus	'1'
	option priority	'2'

config port
        option enable   '1'
        option id       '2'
        option name     'lan2'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '1'
        option id       '3'
        option name     'lan3'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '1'
        option id       '4'
        option name     'lan4'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '0'
        option id       '5'
        option name     'lan5'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '0'
        option id       '6'
        option name     'lan6'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '0'
        option id       '7'
        option name     'lan7'
        option poe_plus '1'
        option priority '2'

config port
        option enable   '0'
        option id       '8'
        option name     'lan8'
        option poe_plus '1'
        option priority '2'