You need to download the file to the router; try using "wget" (replace with "curl" if that fails):
wget https://github.com/eduperez/mwlwifi/releases/download/bc1f345/kmod-mwlwifi_4.4.61.10.3.4.0-20170421-bc1f345-1_arm_cortex-a9_vfpv3.ipk
opkg install kmod-mwlwifi_4.4.61.10.3.4.0-20170421-bc1f345-1_arm_cortex-a9_vfpv3.ipk