Add correct support for Rane SL-1

[GitHub pull_request - opened]

alsa-project/alsa-ucm-conf pull request #187 was opened from christophehenry:

Following [this ticket](https://bugzilla.kernel.org/show_bug.cgi?id=216082) on kernel bugzilla, I'm trying to add corect support for [Rane SL-1](https://cdn.korn.eu/pictures/product/400/040506_3.jpg), an old USB soundcard featuring 2 stereo inputs (RCA) and 3 stereo outputs (2 RCA, 1 6.3mm jack).

Currently, `alsa-info.sh` does show the correct number of inputs and outputs But neither `pavucontrol` nor Mixxx do. I was told complex hardware should be described via UCM but the syntax is unfortunately very poorly documented so I can get my head around it and get `pavucontrol` to show all the inputs and outputs.

Here's what I have so far which, to my disappointment, does not work. If I put only 1 `SectionDevice` section in the file, I can display 1 ouput under `pavucontrol`. With 2 `SectionDevice`, nothing shows.

Without any UCM file, `pavucontrol` shows 2 outputs, 1 intputs (instead of 3) and Mixxx shows 2 outputs and not input.

Request URL   : https://github.com/alsa-project/alsa-ucm-conf/pull/187
Patch URL     : https://github.com/alsa-project/alsa-ucm-conf/pull/187.patch
Repository URL: https://github.com/alsa-project/alsa-ucm-conf