staging: wfx: fix atomic accesses in wfx_tx_queue_empty()

Checking if a skb_queue is empty is not an atomic operation. We should
take some precautions to do it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20210913130203.1903622-8-Jerome.Pouiller@silabs.com


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