crypto: hisilicon/hpre - HPRE_OVERTIME_THRHLD can be written by debugfs

Registers in "hpre_dfx_files" can only be cleaned to zero but
HPRE_OVERTIME_THRHLD, which can be written as any number.

Fixes: 64a6301e("crypto: hisilicon/hpre - add debugfs for ...")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Meng Yu <yumeng18@huawei.com>
Reviewed-by: Zaibo Xu <xuzaibo@huawei.com>
Reviewed-by: Zhou Wang <wangzhou1@hisilicon.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>