media: mc: Rename pad variable to clarify intent

The pad local variable in the media_pipeline_explore_next_link()
function is used to store the pad through which the entity has been
reached. Rename it to origin to reflect that and make the code easier to
read. This will be even more important in subsequent commits when
expanding the function with additional logic.

Cc: stable@vger.kernel.org # 6.1
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>