mlxsw: spectrum_span: Support mirror to ip6gretap

Similarly to mirror-to-gretap, this enables mirroring to IPv6 gretap
netdevice.

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>