crypto: hisilicon/hpre - ensure private key less than n

The private key of the curve key size generated by stdrng, which maybe
not less than n. Therefore, the private key with the curve key size
minus 1 is generated to ensure that the private key is less than n.

Signed-off-by: Weili Qian <qianweili@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>