mt76: mt7921: refactor mt7921_mcu_send_message

This is a preliminary patch to introduce mt7921s support.

Refactor mt7921_mcu_send_message to be sharable 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>