mt76: Introduce mt76_mcu data structure

Introduce mt76_mcu data structure to contain common fields between
mt76u_mcu and mt76e_mcu.
Initialize mcu common fields in mt76_alloc_device().
Rely on mt76_mcu in mt76_mcu_rx_event and in mt76_mcu_get_response
in order to reuse them in usb code

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>