Stop rotor when USB connections break

src/usb.rs

@@ -159,6 +159,8 @@ pub async fn usb_task(
                 buffer = String::from(&buffer.as_str()[line_end + 1..]);
             }
             defmt::info!("USB disconnected");
+            // USB connection is broken, so better stop the rotor.
+            cmd_sender.send(Gs232Cmd::Stop);
         }
     };