mlxsw: spectrum_router: Use correct config option

I made an embarrassing mistake and used 'IPV6' instead of 'CONFIG_IPV6'
around the function that updates the kernel about IPv6 neighbours
activity. This can be a problem if the kernel has more neighbours than a
certain threshold and it starts deleting those that are supposedly
inactive.

Fixes: b5f3e0d43012 ("mlxsw: spectrum_router: Fix build when IPv6 isn't enabled")
Signed-off-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

@@ -1042,7 +1042,7 @@ static void mlxsw_sp_router_neigh_ent_ipv4_process(struct mlxsw_sp *mlxsw_sp,
 	neigh_release(n);
 }
 
-#if IS_ENABLED(IPV6)
+#if IS_ENABLED(CONFIG_IPV6)
 static void mlxsw_sp_router_neigh_ent_ipv6_process(struct mlxsw_sp *mlxsw_sp,
 						   char *rauhtd_pl,
 						   int rec_index)