octeontx2-af: Fix uninitialized variable in debugfs

If rvu_get_blkaddr() fails, then this rvu_cgx_nix_cuml_stats() returns
zero and we write some uninitialized data into the debugfs output.

On the error paths, the use of the uninitialized "*stat" is harmless,
but it will lead to a Smatch warning (static analysis) and a UBSan
warning (runtime analysis) so we should prevent that as well.

Fixes: f967488d ("octeontx2-af: Add per CGX port level NIX Rx/Tx counters")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>