message: core: fix MAVLink FTP bug
  It turns out that O_RDONLY is just 0, so we can't check for the bit as
  we would expect it but we need to check the first two bits to be 0.
diff: @@ -1045,7 +1045,7 @@ MavlinkFtp::ServerResult MavlinkFtp::_work_open(PayloadHeader* payload, int ofla
  }
  // fail only if requested open for read
  - if ((oflag & O_RDONLY) && !fs_exists(path)) {
  + if ((oflag & O_ACCMODE) == O_RDONLY && !fs_exists(path)) {
  LogWarn() << "FTP: Open failed - file not found";
  return ServerResult::ERR_FAIL_FILE_DOES_NOT_EXIST;
  }
  