[media] media.h: don't use legacy entity macros at Kernel

Put the legacy MEDIA_ENT_* macros under a #ifndef __KERNEL__,
in order to be sure that none of those old symbols are used
inside the Kernel.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>