ASoC/mpc5200: add to_psc_dma_stream() helper

Move the resolving of the psc_dma_stream pointer to a helper function
to reduce duplicate code

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>