mt76: usb: use common helpers for mcu_alloc_msg()/mcu_send_msg()

Use mcu common helpers instead of usb specific routines.
Add static qualifier to the following functions:
- mt76u_mcu_msg_alloc
- __mt76u_mcu_send_msg
- mt76u_mcu_send_msg
- mt76u_mcu_wr_rp
- mt76u_mcu_rd_rp
- mt76u_wr_rp
- mt76u_rd_rp
This is a preliminary patch to move mt76x02 usb mcu code in
mt76x02-usb module

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