audio: simplify alt settings management

[HiFiPhile]

Previously alt settings are saved in an array sized by CFG_TUD_AUDIO_FUNC_n_N_AS_INT, since we limit each audio instance to one IN EP and one OUT EP, we can save directly the current alt setting value in the instance, which avoid a lookup operation.

• No more need to define CFG_TUD_AUDIO_FUNC_1_N_AS_INT
• No more need to call audiod_get_AS_interface_index in each transfer, on STM32F7 this call cost 600 cycles = 3us !
• Remaining usage of audiod_get_AS_interface_index_global is replaced by audiod_verify_itf_exists

PS:
In current state it looks like adding UAC1 compatibility is not too complicated, most code should be already compatible.

I've tested on Windows, you can test more if I didn't break anything @PanRe @kf6gpe @gmikitiuk @mastupristi

[Copilot]

Pull Request Overview

This PR simplifies alternate setting management in the TinyUSB audio device class by removing the array-based storage and using direct field storage instead. The change eliminates the need for CFG_TUD_AUDIO_FUNC_n_N_AS_INT configuration and removes lookup operations that were costing 600 cycles (3µs) on STM32F7.

Key Changes

• Replaced array-based alternate setting storage with direct fields in the audio instance structure
• Removed configuration requirements for CFG_TUD_AUDIO_FUNC_n_N_AS_INT across all examples
• Eliminated audiod_get_AS_interface_index function calls that required expensive lookups

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

File	Description
src/class/audio/audio_device.h	Removes configuration validation for CFG_TUD_AUDIO_FUNC_n_N_AS_INT
src/class/audio/audio_device.c	Core implementation changes - adds direct alt setting fields, removes lookup functions
examples/device/*/src/tusb_config.h	Removes CFG_TUD_AUDIO_FUNC_1_N_AS_INT configuration from all audio examples

[hathach]

thank you