Read sample rate directly from control transfer to eliminate callback.

examples/device/audio_4_channel_mic/src/main.c

@@ -407,9 +407,8 @@ bool tud_audio_get_req_entity_cb(uint8_t rhport, tusb_control_request_t const *
         {
           case AUDIO_CS_REQ_CUR:
             TU_LOG2("    Get Sample Freq.\r\n");
-            // Set sample rate for flow control
-            tud_audio_set_tx_flow_control(sampFreq);
-            return tud_control_xfer(rhport, p_request, &sampFreq, sizeof(sampFreq));
+            // Buffered control transfer is needed for IN flow control to work
+            return tud_audio_buffer_and_schedule_control_xfer(rhport, p_request, &sampFreq, sizeof(sampFreq));
 
           case AUDIO_CS_REQ_RANGE:
             TU_LOG2("    Get Sample Freq. range\r\n");