Refactor TUH_AUDIO API and simplify multi-AS interface support

This commit refactors the TUH_AUDIO (USB Audio Host) class driver to
simplify its public API and improve multi-AS (Audio Streaming) interface
support. The changes are focused on three files: the core driver
(audio_host.c/h) and the example application (audio_app.c).

Key changes in src/class/audio/audio_host.h:
- Remove tuh_audio_descriptor_cb_t and tuh_audio_mount_cb_t structures.
  The mount callback no longer passes a large descriptor-info struct;
  applications query per-AS info via tuh_audio_as_get_info().
- Add tuh_audio_get_dev_addr() and tuh_audio_get_feature_unit_id()
  accessors to retrieve device address and feature-unit ID from an
  interface index.
- Simplify control-transfer APIs by replacing (daddr, itf_num, unit_id)
  parameters with a single idx parameter:
    tuh_audio_set_sampling_freq(idx, as_idx, ...)
    tuh_audio_get_sampling_freq(idx, as_idx, ...)
    tuh_audio_feature_unit_set(idx, control_selector, channel, ...)
    tuh_audio_feature_unit_get(idx, control_selector, channel, ...)
- Add synchronous wrapper APIs using TU_API_SYNC macro:
    tuh_audio_get_sampling_freq_sync()
    tuh_audio_set_sampling_freq_sync()
    tuh_audio_feature_unit_set_sync()
    tuh_audio_feature_unit_get_sync()
- Update isochronous endpoint APIs to use (idx, as_idx) instead of
  (daddr, idx):
    tuh_audio_receive(idx, as_idx, buffer, len)
    tuh_audio_send(idx, as_idx, buffer, len)
- Remove tuh_audio_descriptor_cb() weak callback.
- Update tuh_audio_mount_cb() signature from mount_cb(param) to no param.
- Update tuh_audio_rx_cb()/tuh_audio_tx_cb() first parameter from idx to
  dev_addr for consistency with other class drivers.

Key changes in src/class/audio/audio_host.c:
- Delete tuh_audio_descriptor_cb weak stub.
- Refactor get_idx_by_ep_addr() to iterate all AS interfaces per device
  instead of relying on single ep_in/ep_out fields.
- Add audioh_get_ep_addr_by_dir() helper to find an endpoint address by
  direction across multiple AS interfaces.
- Simplify audioh_close() cleanup: remove now-removed single-endpoint
  fields (ep_in, ep_out) and rely on tu_memclr(p_audio->as, ...).
- Update audioh_xfer_cb() to pass dev_addr (not idx) to rx/tx callbacks,
  matching the new callback signature.
- Simplify audioh_open(): remove descriptor-callback emission and the
  temporary desc_cb structure; store only ac_itf_num instead of
  bInterfaceNumber + iInterface + as_interface_num.
- Rename local descriptor pointers for clarity:
    desc_input_terminal (was desc_it)
    desc_output_terminal (was desc_ot)

Key changes in examples/host/audio_host/src/audio_app.c:
- Remove now-unnecessary globals: audio_ep_in, audio_ep_out, audio_ac_itf,
  audio_feature_unit_id.
- Initialize audio_dev_addr, audio_idx, audiostream_in_idx,
  audiostream_out_idx to 0xFF (TUSB_INDEX_INVALID_8) instead of 0.
- Update print_as_interfaces() to use tuh_audio_as_get_count() and
  tuh_audio_as_get_info() instead of accessing mount_cb_data.
- Update all callback signatures and API calls to match the new driver API.

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