hinic: fix sending mailbox timeout in aeq event work

When sending mailbox in the work of aeq event, another aeq event
will be triggered. because the last aeq work is not exited and only
one work can be excuted simultaneously in the same workqueue, mailbox
sending function will return failure of timeout. We create and use
another workqueue to fix this.

Signed-off-by: Luo bin <luobin9@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>