hdspmixer: Hardware output loopback toggle buttons

[GitHub pull_request - reopened]

alsa-project/alsa-tools pull request #4 was reopened from pfandl:

Hi, as promised in the [sound/pci/rme9652 patch](https://github.com/tiwai/sound/commit/da2a040ee7cfe1dd57d5bec7906cb979c5787a86) here are the user land changes which provide the user the ability to toggle the hardware output loopback functionality per output channel. I tried to hide the buttons for devices which do not expose the snd controls yet, but due to probably unnecessary struggle i cancelled that. Please let me know though should you want them removed for not yet enabled devices.

Request URL   : https://github.com/alsa-project/alsa-tools/pull/4
Patch URL     : https://github.com/alsa-project/alsa-tools/pull/4.patch
Repository URL: https://github.com/alsa-project/alsa-tools