[media] tuner/xc2028: Fix frequency offset for radio mode

In radio mode, no frequency offset should be used.

Instead of taking Thierry's patch that creates a separate function
to calculate the digital offset, it seemed better to just keep
everything at the same place.

Reported-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>