mt76: mt7615: move mt7615_mcu_set_bmc to mt7615_mcu_ops

Move mt7615_mcu_set_bmc for fw version 1 and version 2 in
mt7615_mcu_ops data structure. This is a preliminary patch
to properly support mt7663e firmware.
Rework utility routines to rely on skb APIs for msg parsing

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>