net: hns3: refactor dump serv info of debugfs

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

The display style is below:
$ cat service_task_info
local_clock: [  114.203321]
delta: 784(ms)
last_service_task_processed: 4294918512(jiffies)
last_service_task_cnt: 4

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>