mlxsw: spectrum: Move mlxsw_sp_ipip_netdev_{s, d}addr{, 4}()

These functions ideologically belong to the IPIP module, and some
follow-up work will benefit from their presence there.

Signed-off-by: Petr Machata <petrm@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>