V4L/DVB (11918): tuner-xc2028: Fix offset frequencies for DVB @ 6MHz

Both ATSC and DVB @ 6MHz bandwidth require the same offset.

While we're fixing it, let's cleanup the bandwidth setup to better
reflect the fact that it is a function of the bandwidth.

Thanks to Terry Wu <terrywu2009@gmail.com> for pointing this issue and
to Andy Walls <awalls@radix.net> for an initial patch for this fix.

Cc: Terry Wu <terrywu2009@gmail.com>
Acked-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>