crypto: hisilicon/hpre - fix ecdh self test issue

mainline inclusion
from mainline-master
commit 1e609f5f
category: bugfix
bugzilla: 173981
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1e609f5fb73b6b17af369a031f3a4c2b9b405854



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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>
Signed-off-by: Mingqiang Ling <lingmingqiang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>