media: ivsc: csi: Obtain link frequency from the media pad

Support the use of the media pad for obtaining the link frequency.
Similarly, call the v4l2_get_link_freq() on the media pad, not on the
remote's control handler.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>