aoe: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly cancel aoedev->work on free instead of depending on
flush_scheduled_works().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Ed L. Cashin" <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>