crypto: cavium/nitrox - Fix an error handling path in 'nitrox_probe()'

'err' is known to be 0 at this point.
If 'kzalloc()' fails, returns -ENOMEM instead of 0 which means success.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>