[media] em28xx: make sure that the device has video

There are some devices, like Terratec Cinergy HTC, where
while the device supports analog TV, the driver is not
capable yet of handling it, because the analog TV driver
was not written.

So, don't bind the em28xx-v4l drivers on such devices.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>