mt76: usb: move mt76x02 mcu code in mt76x02-usb module

Introduce mt76x02_usb_mcu.c in order to contain mt76x02u mcu related
code. Add mt76x02-usb module as a container for mt76x02 usb code.
This is a preliminary patch to move MT_TXD_INFO, MT_MCU_MSG and
MT_RX_FCE_INFO defs in mt76x02-lib module since other chipsets (e.g.
mt7603) use different dma definitions

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