mtd: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly flush cxt->work_{erase|write} on removal instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>