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.