media: sun4i_csi: Implement link validate for sun4i_csi subdev

The sun4i_csi driver doesn't implement link validation for the subdev it
registers, leaving the link between the subdev and its source
unvalidated. Fix it, using the v4l2_subdev_link_validate() helper.

Fixes: 577bbf23 ("media: sunxi: Add A10 CSI driver")
Cc: stable@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>