mmal/codec: Fix flushing

There is no reason to disable ports before flushing, neither is there a
reason to reconfigure the port after flushing. In fact this lead to
weird issues with the whole mmal stack when the codec was flushed.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>