datapath: Set datapath device MTU to minimum of MTU of ports.

datapath: Set datapath device MTU to minimum of MTU of ports.

The MTU of the local port should be no larger than the minimum of
the MTUs of the ports attached to the bridge, overwise packets may be
dropped.  We already prevent changes to the MTU that would violate
this constraint but don't actuallly proactively set the MTU.  This
changes makes everything consistent and matches the behavior of
the bridge.