octeontx2-af: Modify the return code for unsupported flow keys

The mbox handler npc_install_flow returns ENOTSUPP for unsupported
flow keys. This patch modifies the return value to AF driver defined
error code for debugging purpose.

Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>