V4L/DVB (9972): v4l: usbvideo, fix module ref count check

usbvideo_ClientIncModCount may return value < 0 in the case of error, not > 0.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>