net: hns3: fix loopback failed when phy has no .set_loopback interface

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

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

When phy has no .set_loopback interface, phy loopback test will
return not supported, and the loopback test will failed. So when
phy has no .set_loopback interface, don't do phy loopback test.

Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: Liubin Shu <shuliubin@huawei.com>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>