[ONOS-6414] use Config object from NetworkConfigEvent to avoid NPE

Change-Id: If7170e7b8c18c43321e18c505a15627f485165b6
diff --git a/core/net/src/main/java/org/onosproject/net/link/impl/LinkManager.java b/core/net/src/main/java/org/onosproject/net/link/impl/LinkManager.java
index 3027334..667b94e 100644
--- a/core/net/src/main/java/org/onosproject/net/link/impl/LinkManager.java
+++ b/core/net/src/main/java/org/onosproject/net/link/impl/LinkManager.java
@@ -336,7 +336,7 @@
         @Override
         public void event(NetworkConfigEvent event) {
             LinkKey lk = (LinkKey) event.subject();
-            BasicLinkConfig cfg = networkConfigService.getConfig(lk, BasicLinkConfig.class);
+            BasicLinkConfig cfg = (BasicLinkConfig) event.config().get();
 
             log.debug("Detected link network config event {}", event.type());