NAME
       Flush -- clear all queued I/O requests for the serial port

   FUNCTION
       This command purges the read and write request queues for the
       serial device. Flush will not affect active requests.

   IO REQUEST
       io_Message      mn_ReplyPort initialized
       io_Device       set by OpenDevice
       io_Unit         set by OpenDevice
       io_Command      CMD_FLUSH

   RESULTS
       Error -- if the Flush succeded, then io_Error will be null.
           	 If the Flush failed, then the io_Error will be non-zero.