net: hns3: fix ret not initialized problem in hclge_get_dfx_reg()

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

----------------------------

If the parameter is not valid in hclge_get_dfx_reg(), the ret
is used but not initialized.

Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: li yongxin <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>