audio: Implement support for variable EP in flow control fifo level

[gabChouin]

Describe the PR
This PR adds a tud_audio_set_ep_in_target_fifo_size function that allows the user to set the target fifo level of the EP IN flow control.

Additional context
For applications that require dynamic configuration of the audio EP IN, having the target fifo size set to half of the fifo depth can be limiting. In some cases the input latency might be very big. This MR aims to add the ability to configure this value at run time using the tud_audio_set_ep_in_target_fifo_size function.

[HiFiPhile]

Thank you, it's something I've in mind (also for OUT transfer)

[Copilot]

Pull Request Overview

This PR adds FIFO threshold configuration support for USB audio devices, enabling fine-tuned control over both EP IN (transmit) and EP OUT (receive via feedback) FIFO buffer management for better latency control.

• Adds getter/setter API functions for EP IN FIFO threshold configuration
• Extends audio_feedback_params_t struct with fifo_count.fifo_threshold field for EP OUT feedback control
• Updates flow control logic to use configurable thresholds instead of hardcoded half-depth values

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/class/audio/audio_device.h	Adds API function declarations for EP IN FIFO threshold control and extends feedback params struct with threshold field
src/class/audio/audio_device.c	Implements threshold getter/setter functions, updates packet sizing logic to use configurable thresholds, and refactors feedback parameter handling
examples/device/uac2_speaker_fb/src/main.c	Reorganizes sample_rates array scope and adds documentation explaining FIFO threshold usage in feedback callback

[codescene-delta-analysis]

Gates Failed
Prevent hotspot decline (1 hotspot with Complex Method)
Enforce advisory code health rules (1 file with Complex Method)

Gates Passed
2 Quality Gates Passed

See analysis details in CodeScene

Reason for failure

Prevent hotspot decline	Violations	Code Health Impact
audio_device.c	1 rule in this hotspot	3.44 → 3.43	Suppress

Enforce advisory code health rules	Violations	Code Health Impact
audio_device.c	1 advisory rule	3.44 → 3.43	Suppress

Quality Gate Profile: Clean Code Collective
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[HiFiPhile]

I've done the fixup and also added RX FIFO threshold adjustment.