ASoC: compress: Use power efficient workqueue

There is no need for the power down work to be done on a per CPU workqueue
especially considering the fairly long delay before powerdown.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>