ASoC: intel: Replace kthread with work

The usage pattern of kthread worker in Intel SST drivers can be
replaced gracefully with the normal workqueue, which is more light-
weight and easier to manage in general.  Let's do it.

While in the replacement, move the schedule_work() call inside the
spinlock for excluding the race, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Tested-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>