Enable multi-descriptor transmisstion for fragmented mbufs. There

Enable multi-descriptor transmisstion for fragmented mbufs. There
is no more need to defragment mbufs. After transmitting the
multi-fragmented frame, the controller updates only the first
descriptor of multi-descriptor transmission so it's driver's
responsibility to clear OWN bits of remaining descriptor of
multi-descriptor transmission. It seems the controller behaves much
like jme(4) controllers in descriptor handling.

Tested by:	xclin <xclin <> cs dot nctu dot edu dot tw >