net: hns3: set ops to null when unregister ad_dev

The hclge/hclgevf and hns3 module can be unloaded independently,
when hclge/hclgevf unloaded firstly, the ops of ae_dev should
be set to NULL, otherwise it will cause an use-after-free problem.

Fixes: 38caee9d ("net: hns3: Add support of the HNAE3 framework")
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>