crypto: hisilicon/hpre - fix ecdh self test issue

When the key length is zero, use stdrng to generate private key
to pass the crypto ecdh-nist-p256 self test on vector 2.

Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>