[media] tm6000-video.c: warning fix

drivers/media/usb/tm6000/tm6000-video.c: In function '__check_keep_urb':
drivers/media/usb/tm6000/tm6000-video.c:1926:1: warning: return from incompatible pointer type [enabled by default]

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>