[media] dvb_core: estimate bw for all non-terrestial systems

Instead of just estimating the bandwidth for DVB-C annex A/C,
also fill it at the core for ATSC and DVB-C annex B. This
simplifies the logic inside the tuners, as all non-satellite
tuners can just use c->bandwidth_hz for all supported
delivery systems.

It could make sense to latter use it also for satellite
systems, as several DVB-S drivers have their own calculus.
However, on DVB-S2 the bw estimation is a little more complex,
and the existing drivers have some optimized calculus for
bandwidth. So, let's not touch on it for now.

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