wireless: use newly introduced hex_to_bin()

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Corey Thomas <coreythomas@charter.net>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>