[media] media: Rename media_entity_remote_source to media_entity_remote_pad

Function media_entity_remote_source actually returns the remote pad to
the given one, regardless if this is the source or the sink pad.
Name media_entity_remote_pad is more adequate for this function.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>