lib/ofproto: Sync RSTP operational state after configuration changes.

Otherwise the RSTP port operational state could be out of sync until
the next time the port's carrier status changes.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Daniele Venturino <daniele.venturino@m3s.it>