[media] media-device: fix infoleak in ioctl media_enum_entities()

This fixes CVE-2014-1739.

Change-Id: Ie7f5ceb6f8e85943176bb180b273e90e03f551d8
Signed-off-by: Salva Peiró <speiro@ai2.upv.es>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>