net: hns3: fix return value handle issue for hclge_set_loopback()

In current code, it always return 0, even loopback mode setting failed.
It's incorrect. This patch fixes return value handle for loopback test.

Fixes: 0f29fc23 ("net: hns3: Fix for loopback selftest failed problem")
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>