staging: wfx: fix potential vulnerability to spectre

array_index_nospec() should be applied after a bound check.

Fixes: 9bca45f3 ("staging: wfx: allow to send 802.11 frames")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191011164746.2518-1-Jerome.Pouiller@silabs.com


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