net: hns3: rename variable flag in hnae3_unregister_client()

driver inclusion
category: cleanup
bugzilla: NA
CVE: NA

This patch renames the variable flag into existed, as the meaning of
existed is clearer.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Reviewed-by: Zhong Zhaohui <zhongzhaohui@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>