USB-Audio: Add M-Audio M-Track Solo

[GitHub pull_request - opened <github@alsa-project.org>]

alsa-project/alsa-ucm-conf pull request #796 was opened from busybox11:

The M-Audio M-Track Solo (`08bb:29c0`) is a USB audio interface based on the TI PCM2900C codec.

The chip has a stereo ADC with two physical inputs: Input 1 (XLR/TS combo, channel 0) and Input 2 (TS instrument jack, channel 1), each with its own gain control.
However, the default stereo capture node mixes both into a single stereo stream.

This PR adds a new profile via `M-Audio/M-Track-Solo.conf` and `M-Audio/M-Track-Solo-HiFi.conf` to define the device with two distinct mono capture inputs using SplitPCM, while also providing the proper device name in the labels, instead of the generic `PCM2900C Audio CODEC` prefix.

## Hardware

- Device: M-Audio M-Track Solo
- USB: `08bb:29c0` (Texas Instruments PCM2900C)
- Kernel: `snd-usb-audio`
- Mixer: `PCM Playback Volume/Switch` (stereo output), no capture controls

## UCM configuration

- `Speaker`: stereo output (`hw:${CardId},0`)
- `Mic1`: mono input from channel 0 (XLR/TS combo preamp)
- `Line2`: mono input from channel 1 (TS instrument jack)

## Verification

Before: both physical inputs are mixed into a single stereo capture node.
<img width="3440" height="1440" alt="image" src="https://github.com/user-attachments/assets/e461bc4d-dfe0-4569-848b-2e7bd1328f52" />
<img width="3440" height="1440" alt="image" src="https://github.com/user-attachments/assets/e604a8b7-3fad-424d-92d6-a4d87caccbf8" />


After: two independent mono input nodes (`M-Track SOLO Input 1 (Mic)`, `M-Track SOLO Input 2 (Line)`), each selectable individually in PipeWire/PulseAudio.

<img width="3440" height="1440" alt="image" src="https://github.com/user-attachments/assets/08c019ea-164c-4cd1-8301-f1c3ee46d5e4" />
<img width="3440" height="1440" alt="image" src="https://github.com/user-attachments/assets/1ae5e2df-f337-42cd-9e3b-52a1f402032c" />

Request URL   : https://github.com/alsa-project/alsa-ucm-conf/pull/796
Patch URL     : https://github.com/alsa-project/alsa-ucm-conf/pull/796.patch
Repository URL: https://github.com/alsa-project/alsa-ucm-conf