mt76x02u: use usb_bulk_msg to upload firmware

We don't need to send firmware data asynchronously, much simpler is just
use synchronous usb_bulk_msg().

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