Revert "net/mlx4_en: Avoid unregister_netdev at shutdown flow"

Revert "net/mlx4_en: Avoid unregister_netdev at shutdown flow"

This reverts commit 9d76931180557270796f9631e2c79b9c7bb3c9fb.

Using unregister_netdev at shutdown flow prevents calling
the netdev's ndos or trying to access its freed resources.

This fixes crashes like the following:
 Call Trace:
  [<ffffffff81587a6e>] dev_get_phys_port_id+0x1e/0x30
  [<ffffffff815a36ce>] rtnl_fill_ifinfo+0x4be/0xff0
  [<ffffffff815a53f3>] rtmsg_ifinfo_build_skb+0x73/0xe0
  [<ffffffff815a5476>] rtmsg_ifinfo.part.27+0x16/0x50
  [<ffffffff815a54c8>] rtmsg_ifinfo+0x18/0x20
  [<ffffffff8158a6c6>] netdev_state_change+0x46/0x50
  [<ffffffff815a5e78>] linkwatch_do_dev+0x38/0x50
  [<ffffffff815a6165>] __linkwatch_run_queue+0xf5/0x170
  [<ffffffff815a6205>] linkwatch_event+0x25/0x30
  [<ffffffff81099a82>] process_one_work+0x152/0x400
  [<ffffffff8109a325>] worker_thread+0x125/0x4b0
  [<ffffffff8109a200>] ? rescuer_thread+0x350/0x350
  [<ffffffff8109fc6a>] kthread+0xca/0xe0
  [<ffffffff8109fba0>] ? kthread_park+0x60/0x60
  [<ffffffff816a1285>] ret_from_fork+0x25/0x30

Fixes: 9d7693118055 ("net/mlx4_en: Avoid unregister_netdev at shutdown flow")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reported-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reported-by: Steve Wise <swise@opengridcomputing.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>