Add back debug logging

Author: qiluo-msft

src/sonic_ax_impl/mibs/ietf/rfc4363.py

@@ -99,6 +99,7 @@ def update_data(self):
 
             vlanmac = self.fdb_vlanmac(fdb)
             if not vlanmac:
+                mibs.logger.debug("SyncD 'ASIC_DB' includes invalid FDB_ENTRY '{}': failed in fdb_vlanmac().".format(fdb_str))
                 continue
             self.vlanmac_ifindex_map[vlanmac] = port_index
             self.vlanmac_ifindex_list.append(vlanmac)