VOL-729: ADTRAN ONU - Fix fetch of TCONT attribute from MIB datbase

Change-Id: Iff356cf1ee90fae2b2111cd16be79cc4807c41c8
diff --git a/voltha/adapters/adtran_onu/pon_port.py b/voltha/adapters/adtran_onu/pon_port.py
index 06fe0d3..40dee83 100644
--- a/voltha/adapters/adtran_onu/pon_port.py
+++ b/voltha/adapters/adtran_onu/pon_port.py
@@ -329,7 +329,7 @@
 
                 for tcont in self._tconts.itervalues():
                     free_entity_id = next((k for k, v in tcont_idents.items()
-                                          if isinstance(k, int) and v.get('alloc_id', 0) == 0xFFFF), None)
+                                          if isinstance(k, int) and v.get('attributes', {}).get('alloc_id', 0) == 0xFFFF), None)
                     if free_entity_id is None:
                         self.log.error('no-available-tconts')
                         break