mt76x0: usb: move mcu code in usb_mcu.c

Move usb mcu code in a dedicated source file in order to remove
usb dependency from generic source files and improve code
maintainability

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