usb_free_coherent() is safe with NULL addr and this check is
not required.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20200707030905.3123-1-vulab@iscas.ac.cn


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>