staging: keucr: remove needless check before usb_free_coherent()

usb_free_coherent() is safe with NULL addr 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>