[media] dvb: Allow select between DVB-C Annex A and Annex C

[media] dvb: Allow select between DVB-C Annex A and Annex C

DVB-C, as defined by ITU-T J.83 has 3 annexes. The differences between
Annex A and Annex C is that Annex C uses a subset of the modulation
types, and uses a different rolloff factor. A different rolloff means
that the bandwidth required is slicely different, and may affect the
saw filter configuration at the tuners. Also, some demods have different
configurations, depending on using Annex A or Annex C.

So, allow userspace to specify it, by changing the rolloff factor.

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