staging: rtl8723bs: remove null test before kfree

kfree(..) on a NULL pointer is a no-op; the null test here is redundant.
Detected by coccicheck.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>