[media] v4l: of: Correct v4l2_of_parse_endpoint() kernel-doc

The v4l2_of_parse_endpoint function kernel-doc says that the return value
is always 0. But that is not true since the function can fail and a error
negative code is returned on failure. So correct the kernel-doc to match.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>