[media] uvc_entity: initialize return value

drivers/media/video/uvc/uvc_entity.c: In function ‘uvc_mc_register_entities’:
drivers/media/video/uvc/uvc_entity.c:33: warning: ‘ret’ may be used uninitialized in this function

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