[media] radio-isa: add framework for ISA radio drivers

We have quite a few ISA radio drivers, which are all very similar.

This framework makes it possible to reduce the code size of those drivers
and makes it much easier to keep them up to date with the latest V4L2 API
developments.

Drivers rewritten to use this framework fully pass the v4l2-compliance tests
and are properly using the ISA bus (so they can be found under /sys/bus/isa).

It is now also possible to support multiple cards using the same driver
(tested with two radio-gemtek cards).

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