nfp: flower: add qos offload stats request and reply

Add stats request function that sends a stats request message to hw for
a specific police-filter. Process stats reply from hw and update the
stored qos structure.

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>