ofproto: Update only OFPUTIL_PS_LINK_DOWN (not STP) from netdev state.
When a netdev indicates that its state or configuration has changed,
update_port() updates the OpenFlow port to match the changes. However,
this was being taken too far: a netdev does not have an STP state, and a
state change was resetting the STP state of the port. This fixes the
problem.