ramips: fix Netgear R6220 package selection

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: 38bee61d ("ramips: add support for Netgear R6220")
Signed-off-by: Mathias Kresin <dev@kresin.me>