alsa-mixer: add support for Astro A50

This MR adds support for the Astro A50 USB gaming headset. It uses 2 outputs, for voice and multimedia, but unlike the SteelSeries headsets it supports stereo on both.

(edit: already answered) This MR works, but I think I still need some help:

1. The port names in pavucontrol are shown as steelseries-arctis-output-chat-common and steelseries-arctis-output-game-common - how can I change that?
2. The headset is marketed as "3D Audio ready", whatever that means. aplay -L shows some surrondXY devices, but I can't get speaker-test to use them - where should I be looking for help with that?