message: ftp: add O_TRUNC flag if file is created
  This way we match the implementation in PX4 and ArduPilot, and are
  consistent with
diff: @@ -819,7 +819,7 @@ void FtpImpl::process_mavlink_ftp_message(const mavlink_message_t& msg)
  case CMD_CREATE_FILE:
  LogInfo() << "OPC:CMD_CREATE_FILE";
  - error_code = _work_open(payload, O_CREAT | O_WRONLY);
  + error_code = _work_open(payload, O_CREAT | O_TRUNC | O_WRONLY);
  break;
  case CMD_OPEN_FILE_WO:
  