Use mt76 common signature for mt7603_mcu_msg_send. Move skb allocation
in mt7603_mcu_msg_send and remove duplicated code. This is a preliminary
patch for mt7615-mt7603 mcu code unification
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>