media: mc: Rename pad as origin in __media_pipeline_start()

Rename the pad field in __media_pipeline_start() to both better describe
what it is and avoid masking it during the loop.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>