mt76: move mt76{0,2} mcu shared code in mt76x02_mcu.c

Move shared mt76x2/mt76x0 mcu shared code in a common file
and remove duplicated code

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