drivers/net/wireless/ipw2x00/libipw_module.c: remove unnecessary null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: Stanislav Yakovlev <stas.yakovlev@gmail.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: John W. Linville <linville@tuxdriver.com>