ASoC: pxa-pcm-lib: initialize DMA channel to -1

This fixes a warning ("pxa_free_dma: trying to free channel 0 which is
already freed") when a device was opened but the hw_params() call
failed.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>