media: em28xx-cards: disable V4L2 mode for dual tuners

Right now, the code that calculates alternate modes is not ready
for devices with dual tuners. That's ok, as we currently don't
have any such devices, but better to add a warning for such
case, as, if anyone adds such device, the logic will need to
be reviewed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Brad Love <brad@nextdimension.cc>
Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>