mt76: add usb support to mt76 layer

This will be used by drivers for MT76x2u based devices

Tested-by:  <cug_yangyuancong@hotmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>