staging: wfx: take advantage of wfx_tx_queue_empty()

wfx_tx_queues_check_empty() can be slightly simplified by calling
wfx_tx_queue_empty().

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20210913130203.1903622-9-Jerome.Pouiller@silabs.com


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