mlxsw: spectrum_router: Support IPv6 overlay encap

Add the missing bits to recognize IPv6 next hops as IPIP ones to enable
offloading of IPv6 overlay encapsulation.

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>