mt7615: mcu: use standard signature for mt7615_mcu_msg_send

Use mt76 common signature for mt7615_mcu_msg_send. Move skb allocation
in mt7615_mcu_msg_send and remove duplicated code. Remove
__mt7615_mcu_set_wtbl and __mt7615_mcu_set_sta_rec since now are used
just to send mcu msgs.  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>