net: hns3: refactor dump loopback of debugfs

Currently, the debugfs command for loopback is implemented by
"echo xxxx > cmd", and record the information in dmesg. It's
unnecessary and heavy. To improve it, create a single file
"loopback" for it, and query it by command "cat loopback",
return the result to userspace, rather than record in dmesg.

The display style is below:
$ cat loopback
mac id: 0
app loopback: off
serdes serial loopback: off
serdes parallel loopback: off

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>