octeontx2-af: Cleanup nixlf and blkaddr retrieval logic

Cleanedup repititive nixlf and blkaddr retrieving logic
is various mailbox handlers throughout the rvu_nix.c file.

Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>