nfp: abm: allow to opt-out of RED offload

FW team asks to be able to not support RED even if NIC is capable
of buffering for testing and experimentation.  Add an opt-out flag.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>