net: hns: optimizate fmt of snprintf()

It misses string format in function snprintf(), as below:
snprintf(buff, ETH_GSTRING_LEN, g_gmac_stats_string[i].desc);

It needs to add "%s" to fix it as below:
snprintf(buff, ETH_GSTRING_LEN, "%s", g_gmac_stats_string[i].desc);

Signed-off-by: Qianqian Xie <xieqianqian@huawei.com>
Signed-off-by: Kejian Yan <yankejian@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>