ASoC: PXA: Fix oops in __pxa2xx_pcm_prepare

pxa2xx_pcm_hw_free frees dma channel and sets prtd->dma_ch to -1,
but does not set prtd->params to NULL, so if pxa2xx_pcm_hw_params will
be called immediately, it leaves prtd->dma_ch initialized with -1,
and it results in oops in __pxa2xx_pcm_prepare. This bug is triggered
via SDL.

This patch adds check for prtd->dma_ch to __pxa2xx_pcm_prepare and
cleans prtd->params, so now it works properly.
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/arm/pxa2xx-pcm-lib.c

@@ -140,6 +140,9 @@ int __pxa2xx_pcm_prepare(struct snd_pcm_substream *substream)
 	if (!prtd || !prtd->params)
 		return 0;
 
+	if (prtd->dma_ch == -1)
+		return -EINVAL;
+
 	DCSR(prtd->dma_ch) &= ~DCSR_RUN;
 	DCSR(prtd->dma_ch) = 0;
 	DCMD(prtd->dma_ch) = 0;

sound/soc/pxa/pxa2xx-pcm.c

@@ -65,6 +65,7 @@ static int pxa2xx_pcm_hw_free(struct snd_pcm_substream *substream)
 	if (prtd->dma_ch >= 0) {
 		pxa_free_dma(prtd->dma_ch);
 		prtd->dma_ch = -1;
+		prtd->params = NULL;
 	}
 
 	return 0;