nfp: abm: keep track of all RED thresholds

Instead of writing the threshold out when Qdisc is configured
and not remembering it move to a scheme where we remember all
thresholds.  When configuration changes parse the offloaded
Qdiscs and set thresholds appropriately.

This will help future extensions.

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>