bcm2835: only allow stereo if analogue jack is selected

Sending more than 2 channels to videocore while outputting to analogue
mysteriously outputs heavy artifacts. So just paint it over with a
hack: if analogue is explicitly selected as destination, do not
reporting support for anything other than stereo.

I'm not sure how to deal with the auto case (destination 0). There's
probably way to retrieve this and even to listen to plug events, but
I didn't find one yet, and it's probably not worth the trouble. Just
don't use this setting, I guess. Unless you like noise.

Changing the setting while an audio stream is active also doesn't
work properly. We could probably interrupt running streams by
returning ENODEV or using kernel hotplug stuff (maybe), but that
also doesn't seem worth the trouble.