media: intel/ipu6: Obtain link frequency from the remote subdev pad

Obtain the link frequency from the sub-device's pad instead of a control
handler. This allows obtaining it using the get_mbus_config() sub-device
pad op which is the only method supported by the IVSC driver.

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>