alsa-mixer: Pick at most one fallback mapping

Merged Tanu Kaskinen requested to merge tanuk/pulseaudio:pick-just-one-fallback into master

The old behaviour was such that if none of the normal mappings worked, we would probe ALL fallbacks. I don't think that makes sense, and it caused concrete issues: let's say we have a regular stereo mic device, but there's no "front" PCM defined for it. In this situation we would probe the stereo-fallback mapping (which uses "hw" instead of "front"), and it would work, but then we'd also probe the "multichannel-input" mapping, which would also work, so we end up with two mappings that don't have any difference in behaviour.

I think it's better to simply pick the first working fallback and ignore the rest.

Fixes: (issue is marked as confidential due to unreleased hardware)

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