Fix USB Audio on Windows when used with USB Serial

teensy3/usb_desc.c

 
 #define AUDIO_INTERFACE_DESC_POS	KEYMEDIA_INTERFACE_DESC_POS+KEYMEDIA_INTERFACE_DESC_SIZE
 #ifdef  AUDIO_INTERFACE
-#define AUDIO_INTERFACE_DESC_SIZE	9+10+12+9+12+10+9 + 9+9+7+11+9+7 + 9+9+7+11+9+7+9
+#define AUDIO_INTERFACE_DESC_SIZE	8 + 9+10+12+9+12+10+9 + 9+9+7+11+9+7 + 9+9+7+11+9+7+9
 #else
 #define AUDIO_INTERFACE_DESC_SIZE	0
 #endif
 #endif // KEYMEDIA_INTERFACE
 
 #ifdef AUDIO_INTERFACE
+        // interface association descriptor, USB ECN, Table 9-Z
+        8,                                      // bLength
+        11,                                     // bDescriptorType
+        AUDIO_INTERFACE,                        // bFirstInterface
+        3,                                      // bInterfaceCount
+        0x01,                                   // bFunctionClass
+        0x01,                                   // bFunctionSubClass
+        0x00,                                   // bFunctionProtocol
+        0,                                      // iFunction
 	// Standard AudioControl (AC) Interface Descriptor
 	// USB DCD for Audio Devices 1.0, Table 4-1, page 36
 	9,					// bLength