mt76x0: usb: move initialization code in usb.c

Move usb initialization code in mt76x0-usb module in
order to remove usb dependency from generic init code
(not completed yet). Moreover fix a memory leak in usb probe
if mt76x02_wait_for_mac fails

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>