staging: wfx: detect race condition in WEP authentication

Current code has a special case to handle association with WEP. Before
to rework the tx data handling, let's try to detect any possible misuse
of this code.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-9-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>