alsa-mixer: Enable volume control of SteelSeries Arctis stereo output

nostrad requested to merge nostrad/pulseaudio:arctis_volume into master

Since there is now support for specifying the index of an Element, add the same config as is used for the output-mono variant, as they behave the same: One volume control with no support for adjustments to the left and right channels.

Tested on SteelSeries Arctis Pro Wireless

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