iseries_veth: don't use flush_scheduled_work()

flush_scheduled_work() is on its way out.  Remove its usage from
iseries_veth.

* Cancelling a delayed work, queueing it for immediate execution if
  cancelled and then waiting for completion can be done by simply
  calling flush_delayed_work_sync().

* Explicitly cancel cnx->statemachine_wq on module unload.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Santiago Leon <santil@linux.vnet.ibm.com>
Cc: netdev@vger.kernel.org