mt76x0: usb: move firmware loading to usb.c

Firmware loading is usb specific, move it to usb.c file.

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