Add audio LED support - control API and PCM softvol plugin

[GitHub pull_request - opened]

alsa-project/alsa-lib pull request #121 was opened from perexg:

As proposed for the driver, this set of patches can resolve the LED handling issue for the hardware without mixer switches or volume controls (like AMD ACP bridge).

Request URL   : https://github.com/alsa-project/alsa-lib/pull/121
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/121.patch
Repository URL: https://github.com/alsa-project/alsa-lib