clear specific events before modbus master operation.

[nickrunning]

Next modbus ops may failed if we received some data before operation starting.

Signed-off-by: Xia Haiqiang xiahaiqiang@chinayuanwang.com

[nickrunning]

The FreeRTOS port version as flowing:
`/**

• This function is used for clear all events at last modbus operation.
• @note If you are use OS, you can use OS's event mechanism. Otherwise you have to run
• much user custom delay for waiting.
  /
  void eMBMasterEventClear( void )
  {
  xEventGroupClearBits(xModbusMasterOsEvent, / The event group being tested. */
  EV_MASTER_PROCESS_SUCESS | EV_MASTER_ERROR_RESPOND_TIMEOUT
  | EV_MASTER_ERROR_RECEIVE_DATA | EV_MASTER_ERROR_EXECUTE_FUNCTION);
  }
  `