mt76x2: use common helpers for mcu_alloc_msg()/mcu_send_msg()

Use mcu common helpers instead of mt76x2 specific routines for
mcu_alloc_msg()/mcu_send_msg(). This is a preliminary patch to
unify mt76e and mt76u mcu code

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>