nfp: flower: reject tunnel encap with ipv6 outer headers for offloading

This fixes a bug where ipv6 tunnels would report that it is
getting offloaded to hardware but would actually be rejected
by hardware.

Fixes: b27d6a95 ("nfp: compile flower vxlan tunnel set actions")
Signed-off-by: Louis Peens <louis.peens@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>