testusb: Fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1648088171-30912-1-git-send-email-baihaowen@meizu.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>