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

This fixes CVE-2014-1739.

Change-Id: I15c2fe25bf4bf8ec93aadc9d430c854d8bb3c0d2
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>