NAME
       Clear -- clear the serial port buffers

   FUNCTION
       This command resets the serial port's read buffer pointers.

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

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