media: entity: Use dedicated data link iterator

Where iteration over links for an entity is clearly assuming that
all of those links are data links, use the new iterator to guarantee
that assumption is met.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>