bridge: Enable RSTP if both STP and RSTP are enabled.

bridge: Enable RSTP if both STP and RSTP are enabled.

Until now, if both STP and RSTP were enabled, ovs-vswitchd would actually
enable only the one it first noticed to be enabled, and actually turn off
the setting for the other one in the database (!).  This doesn't match
ovs-vswitchd behavior for other contradictory configurations, so this
commit changes its behavior so that, if both are enabled, RSTP takes
precedence.

Reported-by: Daniele Venturino <daniele.venturino@m3s.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ansis Atteka <aatteka@nicira.com>