crypto: drbg - fix error return code

Fix to return a negative error code from the error handling
case instead of 0.

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Acked-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>