Fix ChassisAgentWriteEvent match condition

As the method description says, the ``ChassisAgentWriteEvent`` uses the
``Chassis_Private.chassis`` information to update the local OVN agent
cache. If there is no chassis, the match should not pass.

This method was updated in [1], during the backwards compatibility
removal. This condition was incorrectly refactored.

[1]https://review.opendev.org/c/openstack/neutron/+/904238

Related-Bug: #2002839
Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I4b4768e0ee3c607dcc8511d7f1b5764d1fd0a63a
(cherry picked from commit bf9e039)

Author: ralonsoh

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovsdb_monitor.py

@@ -339,7 +339,8 @@ def match_fn(self, event, row, old=None):
         # On updates to Chassis_Private because the Chassis has been deleted,
         # don't update the AgentCache. We use chassis_private.chassis to return
         # data about the agent.
-        return event == self.ROW_CREATE or hasattr(old, 'nb_cfg')
+        return (event == self.ROW_CREATE or
+                (hasattr(old, 'nb_cfg') and row.chassis))
 
     def run(self, event, row, old):
         n_agent.AgentCache().update(ovn_const.OVN_CONTROLLER_AGENT, row,