nfp: flower: use stats array instead of storing stats per flow

Make use of an array stats instead of storing stats per flow which
would require a hash lookup at critical times.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>