staging: wfx: simplify wfx_tx_queues_empty()

Thanks to skb_queue_empty_lockless(), it is not necessary to acquire the
spin_lock before to check if the queue is empty.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-10-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>