net: hns3: fix change RSS 'hfunc' ineffective issue

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

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

When user change rss 'hfunc' without set rss 'hkey' by ethtool
-X command, the driver will ignore the 'hfunc' for the hkey is
NULL. It's unreasonable. So fix it.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: li yongxin <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>