mlxsw: router: Enable neighbors to be created on stacked devices

Make the function mlxsw_router_neigh_construct search the rif according
to the neighbour dev other than the dev that was passed to the ndo, thus
allowing creating neigbhours upon stacked devices.

Fixes: 6cf3c971dc84 ("mlxsw: spectrum_router: Add private neigh table")
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c

@@ -657,7 +657,7 @@ int mlxsw_sp_router_neigh_construct(struct net_device *dev,
 		return 0;
 	}
 
-	r = mlxsw_sp_rif_find_by_dev(mlxsw_sp, dev);
+	r = mlxsw_sp_rif_find_by_dev(mlxsw_sp, n->dev);
 	if (WARN_ON(!r))
 		return -EINVAL;