staging: rtl8723au: Remove unnecessary null test before kfree

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

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>