[media] xc5000: Improve it to work better with 6MHz-spaced channels

Brazil uses 6MHz-spaced channels. So, the nyquist filter for
DVB-C should be different, otherwise, inter-channel interference
may badly affect the device, and signal may not be properly decoded.

On my tests here, without this patch, sometimes channels are seen,
but, most of the time, PID filter returns with timeout.

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