mt76: split __mt76u_mcu_send_msg and mt76u_mcu_send_msg routines

Split __mt76u_mcu_send_msg and mt76u_mcu_send_msg in order to be reused
by mt76x0 driver in usb mcu layer unification between mt76x0 and
mt76x2u drivers

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>