mt76: add mt76_mcu_ops data structure for mcu related pointers

Introduce mt76_mcu_ops data structure to contain mcu related function
pointers. This is a preliminary patch to move mt76x02 usb mcu code in
mt76x02-usb module

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