USB-Audio: add Steinberg UR44C profile

alsa-project/alsa-ucm-conf pull request #835 was opened from polyv0x:

## Summary

- stop matching the UR44C to the incompatible four-playback/six-capture
  UR44 profile
- add a dedicated six-playback/eight-capture UR44C UCM profile
- expose split HiFi PCMs for DAW Mix 1, DAW Mix 2, FX Bus, Direct Out
  1-6, and Streaming L/R

The channel layout comes from the device's USB descriptors:

| Direction | Channels |
| --- | --- |
| Playback | DAW Mix1 L/R, DAW Mix2 L/R, FX Bus L/R |
| Capture | DIRECT OUT 1-6, Streaming L/R |

This retains ACP's Pro Audio profile while also providing working HiFi and
Direct UCM profiles instead of only falling back to ACP.

## Testing

- `ucm-validator2`: `make configs` passes for the complete configuration tree
- `alsaucm -c hw:0 dump text` loads the profile against the connected UR44C
- WirePlumber 0.5.15 / PipeWire 1.6.8 exposes:
  - HiFi: 3 sinks and 10 sources
  - Direct: 1 six-channel sink and 1 eight-channel source
- activated HiFi and verified every split node was instantiated, then switched
  back to Pro Audio
- no UCM or channel-count errors in the WirePlumber log

[alsa-info for the tested UR44C](https://gist.github.com/polyv0x/8b95168b5705d141a9910d56a979be78)

Fixes #799

Request URL   : https://github.com/alsa-project/alsa-ucm-conf/pull/835
Patch URL     : https://github.com/alsa-project/alsa-ucm-conf/pull/835.patch
Repository URL: https://github.com/alsa-project/alsa-ucm-conf