mt76: mt7921: refactor mcu.c to be bus independent

This is a preliminary patch to introduce mt7921s support.

Make mcu.c reusable between mt7921s and mt7921e

Tested-by: Deren Wu <deren.wu@mediatek.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>