ALSA: Restore support for DMAless DAIs on PXA

Used for applications such as direct bluetooth connections on
smartphones which don't go via the CPU. This used to be supported
before the refactoring to share code but this check was removed
during that move.
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

sound/arm/pxa2xx-pcm-lib.c

@@ -136,6 +136,9 @@ int __pxa2xx_pcm_prepare(struct snd_pcm_substream *substream)
 {
 	struct pxa2xx_runtime_data *prtd = substream->runtime->private_data;
 
+	if (!prtd || !prtd->params)
+		return 0;
+
 	DCSR(prtd->dma_ch) &= ~DCSR_RUN;
 	DCSR(prtd->dma_ch) = 0;
 	DCMD(prtd->dma_ch) = 0;