media: microchip-isc: Drop v4l2_subdev_link_validate() for video devices

The v4l2_subdev_link_validate() function is a helper designed to
validate links whose sink is a subdev. When called on a link whose sink
is a video device, it only prints a warning and returns. As the
microchip-isc driver implements manual validate of the subdev to video
device link, we can just drop the v4l2_subdev_link_validate() to avoid
the warning.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>