nfp: flower: move non-zero chain check

This is in preparation for conntrack offload support which makes
used of different chains. Add explicit checks for conntrack and
non-zero chains in the add_offload path.

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>