nfp: flower-ct: fix error return code in nfp_fl_ct_add_offload()

If nfp_tunnel_add_ipv6_off() fails, it should return error code
in nfp_fl_ct_add_offload().

Fixes: 5a2b9304

 ("nfp: flower-ct: compile match sections of flow_payload")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>