staging: wfx: avoid useless wake_up

staging: wfx: avoid useless wake_up

__wfx_flush() wait for all queues to be empty. In current code,
wait_link_id_empty is wake up each time there is no more data for a
station. We can simplify the processing and avoid some wake-up by
raising this event only when 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/20200401110405.80282-8-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>