[media] tuner, xc2028: add support for get_afc()

Implement API support to return AFC frequency shift, as this device
supports it. The only other driver that implements it is tda9887,
and the frequency there is reported in Hz. So, use Hz also for this
tuner.

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