net: hns3: refactor dump mng tbl of debugfs

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

The display style is below:
$ cat mng_tbl
entry  mac_addr          mask  ether  mask  vlan  mask  i_map ...
00     00:00:00:00:00:00 0     88cc   0     0000  1     0f    ...

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>