[media] mt2031: remove fake implementaion of get_bandwidth()

[media] mt2031: remove fake implementaion of get_bandwidth()

This driver implements a fake get_bandwidth() callback. In
reallity, the tuner driver won't adjust its low-pass
filter based on a bandwidth, and were just providing a fake
method for demods to read whatever was "set".

This code is useless, as none of the drivers that use
this tuner seems to require a get_bandwidth() callback.

While here, convert set_params to use the DVBv5 way to pass
parameters to tuners.

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