media: mc: Make media_get_pad_index() use pad type flag

Use the pad flag specifying the pad type instead of a boolean in
preparation for internal source pads.

Also make the loop variable unsigned.

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-cisco@xs4all.nl>