Dear Maintainer,

After upgrading linux-image-amd64 from 6.12.113-1 to 6.12.115-2,
audio playback through my Steinberg UR44C USB audio interface
stopped working. Booting back into 6.12.113 restores playback
immediately, with no other changes.

Hardware: Steinberg UR44C (Yamaha Corporation), USB ID 0499:1730,
connected to a rear USB port (high speed). The device uses
implicit feedback mode. Power is supplied via a wall plug, not USB.

Audio stack: PipeWire 1.4.2 / WirePlumber, card set to the
"Pro Audio" profile. No changes to the audio packages in the
last two months; the only relevant upgrade was the kernel.

Symptoms on 6.12.115:
 - Applications (Firefox, fluidsynth) connect to the UR44C
   playback ports and show as active, but no sound is produced.
 - The playback and capture streams in
   /proc/asound/UR44C/stream0 show "Status: Running".
 - PipeWire logs repeatedly:
     spa.audioconvert: (187 suppressed) out of buffers on port 0 1
   meaning the ALSA sink has stopped consuming buffers.

On 6.12.113, the same setup plays normally. The stream0 output
is identical on both kernels:

  Playback: Interface 1, Altset 1, S32_LE, 6 channels,
  Endpoint 0x01 (OUT, ASYNC), Sync Endpoint 0x81,
  Implicit Feedback Mode: Yes, rates 44100-192000.
  Capture: Interface 2, Altset 1, S32_LE, 6 channels,
  Endpoint 0x81 (IN, ASYNC).

Expected: playback works on 6.12.115 as it does on 6.12.113.

Since stream0 and the PipeWire configuration are the same on both
kernels, I suspect a change to snd-usb-audio (possibly the
implicit feedback / endpoint handling) between 6.12.113 and
6.12.115.

Note: WirePlumber also logs an "ALSA UCM error" for this card
(alsa-ucm-conf matches the UR44C to the older UR44 profile, which
expects 4 playback channels, the UR44C has 6). That message appears on both
kernels and is unrelated to this regression.

I'm happy to test patches or candidate kernels.