ofproto: Update only OFPUTIL_PS_LINK_DOWN (not STP) from netdev state.

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.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reported-by: Vasu Dasari <vdasari@gmail.com>
Tested-by: Vasu Dasari <vdasari@gmail.com>