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.