staging: r8188eu: remove needless check before usb_free_urb()

usb_free_urb(NULL) is safe and this check is not required.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>