mt76x2: move mt76x2 mcu shared code in mt76x2_mcu_common.c

Move shared mt76x2 {pcie/usb} 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>