nfp: abm: wrap RED parameters in bands

Wrap RED parameters and stats into a structure, and a 1-element
array.  Upcoming GRED offload will add the support for more bands.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>