V4L/DVB (6683): Fix DVB compatibility

DVB-S is not supported. Also, there are some QAM6 firmwares for xc3028, but it
is reported that this doesn't work fine.

Thanks to Manu Abraham, Michael Krufky and Patrick Boettcher for their
insights.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>