mt76: introduce __mt76_mcu_send_firmware routine

Introduce __mt76_mcu_send_firmware routine to specify mcu message max
length. This is a preliminary patch to support mt7921s driver.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>