[media] media: Move MEDIA_ENTITY_MAX_PADS from media-entity.h to media-entity.c

This isn't really a part of any interface drivers are expected to use. In
order to keep drivers from using it, hide it in media-entity.c. This was
always an arbitrary number and should be removed in the long run.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>