crypto: drbg - fix an error code in drbg_init_sym_kernel()

We accidentally return PTR_ERR(NULL) which is success but we should
return -ENOMEM.

Fixes: 35591285

 ('crypto: drbg - use CTR AES instead of ECB AES')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>