Fix USB Serial break on Teensy3 (needed by Labview)

http://forum.pjrc.com/threads/25350-Teensy-3-1-Serial-Break

teensy3/usb_dev.c

 		usb_cdc_line_rtsdtr = setup.wValue;
 		//serial_print("set control line state\n");
 		break;
+	  case 0x2321: // CDC_SEND_BREAK
+		break;
 	  case 0x2021: // CDC_SET_LINE_CODING
 		//serial_print("set coding, waiting...\n");
 		return;