nfp: add control message passing capabilities to flower offloads

Previously the flower offloads never sends messages to the hardware,
and never registers a handler for receiving messages from hardware.
This patch enables the flower offloads to send control messages to
hardware when adding and removing flow rules. Additionally it
registers a control message rx handler for receiving stats updates
from hardware for each offloaded flow.

Additionally this patch adds 4 control message types; Add, modify and
delete flow, as well as flow stats. It also allows
nfp_flower_cmsg_get_data() to be used outside of cmsg.c.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>