ASoC: tlv320dac33: Change RT wq to singlethread wq

RT workqueue is going away in the near future, replace it with
singlethread wq for now, which is still supported.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>