Because the a52 plugin (being an ioplug) does not proxy snd_pcm_info for
its slave PCM, we need a different way to figure out the card associated
with the device. It turns out we do have access to the card index
already in the caller, so let's pass that index down as a fallback.
This is done as a fallback rather than replacing the existing lookup in
case there are situations where they might be mismatched. We could
possibly just replace the existing lookup, but the cost seems low enough
to not merit the risk of changing this.