ADTRAN ONU: fix assert on ONU reconcile

Change-Id: Idd05432a3c230848d4483f1197595fd65daccafa
diff --git a/voltha/adapters/adtran_onu/pon_port.py b/voltha/adapters/adtran_onu/pon_port.py
index 4962f4e..d82eff9 100644
--- a/voltha/adapters/adtran_onu/pon_port.py
+++ b/voltha/adapters/adtran_onu/pon_port.py
@@ -112,7 +112,7 @@
 
     @entity_id.setter
     def entity_id(self, value):
-        assert self._entity_id is None, 'Cannot reset the Entity ID'
+        assert self._entity_id is None or self._entity_id == value, 'Cannot reset the Entity ID'
         self._entity_id = value
 
     @property