net: hns3: fix missing help info in debugfs

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

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

This patch adds missing help info for dumping MAC umv counter in
debugfs.

Fixes: 0d49a3dea874 ("net: hns3: add support for dumping MAC umv counter in debugfs")
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>