drivers/staging/rtl8821ae/rtl8821ae/hal_btc.c: remove unnecessary null test before kfree

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

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: devel@driverdev.osuosl.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>