mt76: usb: introduce mt76u_fill_bulk_urb routine

Add mt76u_fill_bulk_urb to initialize tx/rx urbs and
remove duplicate code

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