nfp: flower-ct: add delete flow handling for ct

Add functions to handle delete flow callbacks for ct flows. Also
accept the flows for offloading by returning 0 instead of -EOPNOTSUPP.
Flows will still not actually be offloaded to hw, but at this point
it's difficult to not accept the flows and also exercise the cleanup
paths properly. Traffic will still be handled safely through the
fallback 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>