ASoC: sta32x: Use power efficient workqueue

None of the delayed work the driver schedules has particularly short delays
and it is not performance sensitive so let the scheduler run it wherever
is most efficient rather than in a per CPU workqueue by using the system
power efficient workqueue.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>