sound: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.

* cancel[_delayed]_work() + flush_scheduled_work() ->
  cancel[_delayed]_work_sync().

* wm8350, wm8753 and soc-core use custom code to cancel a delayed
  work, execute it immediately if it was pending and wait for its
  completion.  This is equivalent to flush_delayed_work_sync().  Use
  it instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>