[media] media-device: better name Kernelspace/Userspace links

The __media_device_enum_links() copies links definitions from
Kernelspace to userspace. It has to work with 3 structs that
handle with links. Better name them to:
	link: Kernelspace internal link representation, of the
		type media_link;
	klink_desc:  struct media_link_desc pointer to the
		kernel memory where the data will be filled;
	ulink_desc:  struct media_link_desc pointer to the
		memory where the data will be copied to
		userspace.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>