nfp: flower: offload MPLS pop action

Recent additions to the kernel include a TC action module to manipulate
MPLS headers on packets. Such actions are available to offload via the
flow_offload intermediate representation API.

Modify the NFP driver to allow the offload of MPLS pop actions to
firmware. The act_mpls TC module enforces that the next protocol is
supplied along with the pop action. Passing this to firmware allows it
to properly rebuild the underlying packet after the pop.

Signed-off-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>