[media] tuner-core: fix compilation if the media controller is not defined

drivers/media/v4l2-core/tuner-core.c:440:7: error: 'struct v4l2_subdev' has no member named 'entity'
     t->sd.entity.name = t->name;

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>