net: microchip: sparx5: Improve the IP frame key match for IPv6 frames

This ensures that it will be possible for a VCAP rule to distinguish IPv6
frames from non-IP frames, as the IS0 keyset usually selected for the IPv6
traffic class in (7TUPLE) does not offer a key that specifies IPv6
directly: only non-IPv4.

The IP_SNAP key ensures that we select (at least) IP frames.

Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>