mt76: mt7615: introduce MCU_FW_PREFIX for fw mcu commands

Currently fw commands rely on negative cmds since they need different
mcu msg metadata. Extend this approach introducing MCU_FW_PREFIX.
This is a preliminary patch to support new mt7663e firmware commands

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