Fixed that the stp enable column name

The ovs vswitchd manual said that the stp enable column name is stp_enable.
But onos's implementation is stpenable. Please refers to following document.

http://www.openvswitch.org//ovs-vswitchd.conf.db.5.pdf

Change-Id: I10973a7c880d27ccef63b9f2e6730272b4280124

Author: DongRyeol Cha

protocols/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/table/Bridge.java

@@ -36,7 +36,7 @@ public class Bridge extends AbstractOvsdbTableService {
      */
     public enum BridgeColumn {
         NAME("name"), DATAPATHTYPE("datapath_type"), DATAPATHID("datapath_id"),
-        STPENABLE("stpenable"), PORTS("ports"), MIRRORS("mirrors"),
+        STPENABLE("stp_enable"), PORTS("ports"), MIRRORS("mirrors"),
         NETFLOW("netflow"), SFLOW("sflow"), IPFIX("ipfix"),
         CONTROLLER("controller"), PROTOCOLS("protocols"),
         FAILMODE("fail_mode"), STATUS("status"), OTHERCONFIG("other_config"),