net: hns3: refactor dump loopback of debugfs

mainline inclusion
from mainline-master
commit d658ff34
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d658ff34dd7ff9ccf13c59a7c464b55ca122977d



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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>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Junxin Chen <chenjunxin1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>