mt76x0: remove mt76x0_mcu_msg_alloc routine

Remove mt76x0_mcu_msg_alloc duplicated routine and use mt76u_mcu_msg_alloc
utility function for usb mcu message allocation

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