Use isochronous adaptive mode for audio transmit

teensy3/usb_desc.c

 	9, 					// bLength
 	5, 					// bDescriptorType, 5 = ENDPOINT_DESCRIPTOR
 	AUDIO_TX_ENDPOINT | 0x80,		// bEndpointAddress
-	0x05, 					// bmAttributes = isochronous, asynchronous
+	0x09, 					// bmAttributes = isochronous, adaptive
 	LSB(AUDIO_TX_SIZE), MSB(AUDIO_TX_SIZE),	// wMaxPacketSize
 	1,			 		// bInterval, 1 = every frame
 	0,					// bRefresh