[media] tuner-core/tda9887: get_afc can be tuner mode specific

The get_afc op in tda9887 is valid only for the radio mode.
But due to the way get_afc in analog_demod_ops was designed it would
overwrite the afc value with a bogus value when in TV mode.
Pass a pointer to the afc value instead, and when not in radio mode
leave it alone in the tda9887.
This broke a long time ago in 2.6.19 when the get_afc op was introduced.
Before that the afc was only set for radio mode in the tda9887.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>