octeontx2-af: Fix possible null pointer dereference.

This patch fixes possible null pointer dereference in files
"rvu_debugfs.c" and "rvu_nix.c"

Fixes: 8756828a ("octeontx2-af: Add NPA aura and pool contexts to debugfs")
Fixes: 9a946def

 ("octeontx2-af: Modify nix_vtag_cfg mailbox to support TX VTAG entries")
Signed-off-by: Rakesh Babu Saladi <rsaladi2@marvell.com>
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>