mt76x2: introduce mt76x2_init_device routine

Add mt76x2_init_device routine in order to reuse common pcie/usb mac80211
initialization code supporting mt76x2u based device

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