The Netgear R6220 requires the kmod-usb3 package and misses
kmod-usb-ledtrig-usbport package to setup the configured usb led
trigger.
Drop the already target selected kmod-mt76.
Fixes: FS#686
Fixes: 38bee61dab ("ramips: add support for Netgear R6220")
Signed-off-by: Mathias Kresin <dev@kresin.me>