Only allow tab completion for one DeviceID in `links` command

Change-Id: I794018b10ebc2070ed1fc7226990aa4efd572c31
diff --git a/cli/src/main/resources/OSGI-INF/blueprint/shell-config.xml b/cli/src/main/resources/OSGI-INF/blueprint/shell-config.xml
index 00ecdee..9033d43 100644
--- a/cli/src/main/resources/OSGI-INF/blueprint/shell-config.xml
+++ b/cli/src/main/resources/OSGI-INF/blueprint/shell-config.xml
@@ -248,6 +248,7 @@
             <action class="org.onosproject.cli.net.LinksListCommand"/>
             <completers>
                 <ref component-id="deviceIdCompleter"/>
+                <null/> <!-- no more arguments -->
             </completers>
         </command>