Override qb_fdb_index() in L3::Dell since the dot1qTpFdbTable uses dot1qVlanIndex rather than dot1qFdbId as an index

Info/Layer3/Dell.pm

@@ -213,6 +213,10 @@ sub _vendor {
     }
 }
 
+# dot1qTpFdbTable uses dot1qVlanIndex rather than dot1qFdbId as index,
+# so pretend we don't have the mapping
+sub qb_fdb_index {return}
+
 1;
 __END__
 
@@ -387,6 +391,10 @@ sometimes not unique.
 Returns reference to hash of iid to current link administrative duplex
 setting.
 
+=item $dell->qb_fdb_index()
+
+Returns nothing to work around incorrect indexing of C<dot1qTpFdbTable>
+
 =back
 
 =head2 Table Methods imported from SNMP::Info::Layer3