Fix issues found by FindBugs: RV_RETURN_VALUE_IGNORED_BAD_PRACTICE

http://findbugs.sourceforge.net/bugDescriptions.html#RV_RETURN_VALUE_IGNORED_BAD_PRACTICE

Note: This fix doesn't address explicitly the potential issue of preventing
a queue from growing infinitely.
This is a generic issue we might have in other parts of the code as well,
and we need to address/evaluate it explicitly.

Change-Id: I6908ba8625af7eda5f889a16245614c7df852cfe
diff --git a/src/main/java/net/onrc/onos/core/registry/ZookeeperRegistry.java b/src/main/java/net/onrc/onos/core/registry/ZookeeperRegistry.java
index e91ab27..464dd01 100755
--- a/src/main/java/net/onrc/onos/core/registry/ZookeeperRegistry.java
+++ b/src/main/java/net/onrc/onos/core/registry/ZookeeperRegistry.java
@@ -150,14 +150,14 @@
         public void isLeader() {
             log.debug("Became leader for {}", dpid);
 
-            switchLeadershipEvents.offer(new SwitchLeaderEvent(HexString.toLong(dpid), true));
+            switchLeadershipEvents.add(new SwitchLeaderEvent(HexString.toLong(dpid), true));
         }
 
         @Override
         public void notLeader() {
             log.debug("Lost leadership for {}", dpid);
 
-            switchLeadershipEvents.offer(new SwitchLeaderEvent(HexString.toLong(dpid), false));
+            switchLeadershipEvents.add(new SwitchLeaderEvent(HexString.toLong(dpid), false));
         }
     }