alsa-mixer: add support for Sennheiser GSX 1000/1200 gaming DACs

Merged Patrick McLean requested to merge chutz/pulseaudio:sennheiser-gsx-1000 into master

This adds support for the Sennheiser GSX 1000/1200 gaming DACs. It mostly works like a gaming USB headset, but it accepts an analog headset rather than being one itself.

Edited by Patrick McLean

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