net/mlx5e: Use ipv6_stub to avoid dependency with ipv6 being a module

mlx5 is dependent on IPv6 tristate since we use ipv6's nd_tbl directly,
alternatively we can use ipv6_stub->nd_tbl and remove the dependency.

Reported-by: Walter Harms <wharms@bfs.de>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>