nfp: flower: update nfp_tun_neigh structs

Prepare for more rework in following patches by updating
the existing nfp_neigh_structs. The update allows for
the same headers to be used for both old and new firmware,
with a slight length adjustment when sending the control message
to the firmware.

Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>