Bug fix when changing ports from access to trunk mode

Don't attempt to add the trunk to the default VLAN; it's already going
to be there.

Change-Id: Id06ddf000eae7d6e5f9725a251dd057a150f9ca6
diff --git a/util.py b/util.py
index a35a6bb..e537256 100644
--- a/util.py
+++ b/util.py
@@ -505,7 +505,8 @@
                 else:
                     vlans = db.all_vlans()
                     for vlan in vlans:
-                        s.port_add_trunk_to_vlan(port.name, vlan.tag)
+                        if vlan != state.config.vland.default_vlan_tag:
+                            s.port_add_trunk_to_vlan(port.name, vlan.tag)
 
         except IOError:
             logging.error('set_port_mode failed, resetting switch to recover')