Fixes to setup_dhcpd change which broke cord-tester.

Change-Id: Ief013a722e0043172712802761d4ed5f90f9addf
diff --git a/src/test/utils/VolthaCtrl.py b/src/test/utils/VolthaCtrl.py
index aa19e6a..e5a996f 100644
--- a/src/test/utils/VolthaCtrl.py
+++ b/src/test/utils/VolthaCtrl.py
@@ -480,7 +480,8 @@
     return None
 
 def voltha_teardown(voltha_ctrl, device_id, switch_map, olt_app = None):
-    voltha_ctrl.disable_device(device_id)
+    if voltha_ctrl:
+        voltha_ctrl.disable_device(device_id)
     time.sleep(10)
     if olt_app is None:
         olt_app = get_olt_app()