octeontx2-af: add proper return codes for AF mailbox handlers

Add appropriate error codes to be used when returning from AF
mailbox handlers due to some error condition.

Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>