media: v4l2-subdev: Drop unreacheable warning

The v4l2_subdev_link_validate_get_format() function warns if the pad
given as argument belongs to a non-subdev entity. This can't happen, as
the function is called from v4l2_subdev_link_validate() only (indirectly
through v4l2_subdev_link_validate_locked()), and that function ensures
that both ends of the link are subdevs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>