For STAC9221 Macs, create master volume and mute controls by binding the DAC volumes.
For STAC9221 Macs, create master volume and mute controls by binding
the DAC volumes.

Tested by otto@, ajacoutot@, merdeley@ and Darrin Chandler.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars