message: mavlink_ftp: just fix max_data_length
  This is required because MSVC complained:
  warning C4200: nonstandard extension used: zero-sized array in
  struct/union
diff: @@ -116,6 +116,10 @@ private:
  static constexpr auto DIRENT_DIR = "D"; ///< Identifies Directory returned from List command
  static constexpr auto DIRENT_SKIP = "S"; ///< Identifies Skipped entry from List command
  +
  + /// @brief Maximum data size in RequestHeader::data
  + static constexpr uint8_t max_data_length = 239;
  +
  /// @brief This is the payload which is in mavlink_file_transfer_protocol_t.payload.
  /// This needs to be packed, because it's typecasted from
  /// mavlink_file_transfer_protocol_t.payload, which starts at a 3 byte offset, causing an
  @@ -130,13 +134,9 @@ private:
  ///< packets complete, 0: More burst packets coming.
  uint8_t padding; ///< 32 bit alignment padding
  uint32_t offset; ///< Offsets for List and Read commands
  - uint8_t data[]; ///< command data, varies by Opcode
  + uint8_t data[max_data_length]; ///< command data, varies by Opcode
  });
  - /// @brief Maximum data size in RequestHeader::data
  - static const uint8_t max_data_length =
  - MAVLINK_MSG_FILE_TRANSFER_PROTOCOL_FIELD_PAYLOAD_LEN - sizeof(PayloadHeader);
  -
  struct SessionInfo {
  int fd{-1};
  uint32_t file_size{0};
  