Add audio LED support - control API and PCM softvol plugin

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