octeontx2-af: Add response for RSS flow key cfg message

Added response for nix_rss_flowkey_cfg message to return
selected RSS algorithm index.

The FLOW_KEY_TYPE* definition is part of the mbox message and
it will be used by the other consumers of AF driver hence moving to mbox.h.

Also renamed FLOW_* definitions to NIX_FLOW_* to avoid global
name space collisions, as we have various coming from
include/uapi/linux/pkt_cls.h for example.

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>