ofproto-dpif: Cleanup STP on ports when disabled on their bridge.

ofproto-dpif: Cleanup STP on ports when disabled on their bridge.

When STP is enabled on a bridge, the STP module puts its ports in
an STP_LISTENING state until STP converges.  This causes all
traffic destined for these ports to be dropped.  If STP is disabled
on the bridge, but not explicitly disabled on its ports, the bridge
fails to remove the STP state from these ports.  Therefore, if a
port is in an STP_LISTENING state, it will remain in that state and
continue to drop all traffic indefinitely.  This patch fixes the
issue.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Bug #9157.