usb_audio: mark speaker OUT adaptive so Windows UAC2 can start

[jbirchall-svg]

Closes: #11216

Fixes Windows 11 Code 10 ("This device cannot start") when usb_audio.enable(speaker=True) is used.

The speaker (and headset speaker) OUT endpoint was copied from TinyUSB's
TUD_AUDIO20_SPEAKER_*_FB_DESCRIPTOR but the feedback endpoint was
intentionally omitted. The data EP was still marked Asynchronous.
usbaudio2.sys requires an explicit feedback endpoint for async OUT and
does not support implicit feedback, so the driver fails during start
before any audio flows. Linux is more lenient, which is why RP2040
speaker tests looked fine there.

The USBSpeaker sink already adapts: it drains whatever the host sends
and underruns/overruns in the ring. Advertise that honestly as Adaptive
instead of Asynchronous. Microphone IN stays Asynchronous (valid
without a feedback EP).

This is not a full clock-matching implementation. Long-term drift can
still produce drop/insert artifacts; a later change can add a feedback
EP (and TinyUSB CFG_TUD_AUDIO_ENABLE_FEEDBACK_EP) if we need the
board to be the clock master. Explicit feedback is also a poor first
fix on full-speed parts: Windows' UAC2 driver has known FS feedback
format bugs, and TinyUSB's own speaker example emits UAC1 on FS.

Test plan:

• Windows 11: usb_audio.enable(sample_rate=48000, channel_count=2, microphone=False, speaker=True) — audio device starts, no Code 10, playback reaches I2S
• Windows 11: usb_audio.enable(speaker=True) (headset; mic still defaults on) — speaker device starts
• Linux: speaker-only and mic-only still enumerate and stream
• nRF52840 speaker-only (forced ISO EP 8) still streams

[dhalbert]

@jbirchall-svg The original post looks like LLM output? Have you done the tests listed?

[jbirchall-svg]

@jbirchall-svg The original post looks like LLM output? Have you done the tests listed?

@dhalbert it is LLM output (Cursor AI Grok 4.6 model) and I have only done the Windows 11 Pro tests on a single Adafruit QT Py RP2040 board connected to an Adafruit I2S Amplifier BFF and an 8 Ohm speaker.

Audio streams correctly for me from the PC, through the circuit, and into the speaker after applying the PR and building my fork.

[FoamyGuy]

I had an agent confirm no regressions from this change on linux.

I don't have any easy way to test on windows.