mt76: usb: reduce locking in mt76u_tx_tasklet

Similar to pci counterpart, reduce locking in mt76u_tx_tasklet since
q->head is managed just in mt76u_tx_tasklet and q->queued is updated
holding q->lock

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>