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: 64a6301("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>

drivers/crypto/hisilicon/hpre/hpre_main.c

@@ -548,13 +548,15 @@ static int hpre_debugfs_atomic64_get(void *data, u64 *val)
 static int hpre_debugfs_atomic64_set(void *data, u64 val)
 {
 	struct hpre_dfx *dfx_item = data;
-	struct hpre_dfx *hpre_dfx = dfx_item - HPRE_OVERTIME_THRHLD;
+	struct hpre_dfx *hpre_dfx = NULL;
 
-	if (val)
+	if (dfx_item->type == HPRE_OVERTIME_THRHLD) {
+		hpre_dfx = dfx_item - HPRE_OVERTIME_THRHLD;
+		atomic64_set(&hpre_dfx[HPRE_OVER_THRHLD_CNT].value, 0);
+	} else if (val) {
 		return -EINVAL;
+	}
 
-	if (dfx_item->type == HPRE_OVERTIME_THRHLD)
-		atomic64_set(&hpre_dfx[HPRE_OVER_THRHLD_CNT].value, 0);
 	atomic64_set(&dfx_item->value, val);
 
 	return 0;