[media] radio-mr800: add support for stereo and signal detection

Thanks to an older driver by Faidon Liambotis <paravoid@debian.org> (as noted
in the radio-mr800 comment block at the start) for figuring out how to get the
signal/stereo state.

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