flow_offload: Introduce offload of HW stats type

Initially, pass "ANY" (struct is zeroed) to the drivers as that is the
current implicit value coming down to flow_offload. Add a bool
indicating that entries have mixed HW stats type.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>