net: hns3: fix use-after-free bug in hclgevf_send_mbx_msg

Currently, the hns3_remove function firstly uninstall client instance,
and then uninstall acceletion engine device. The netdevice is freed in
client instance uninstall process, but acceletion engine device uninstall
process still use it to trace runtime information. This causes a use after
free problem.

So fixes it by check the instance register state to avoid use after free.

Fixes: d8355240 ("net: hns3: add trace event support for PF/VF mailbox")
Signed-off-by: Jie Wang <wangjie125@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>