ASoC: check for substream not channels_min in pcm engines

This is a follow up on 53dea36c70c1857 which fixes the other affected
pcm engines.

Description from 53dea36c70c1857:
 Don't rely on the codec's channels_min information to decide wheter or
 not allocate a substream's DMA buffer. Rather check if the substream
 itself was allocated previously.

Without this patch I was seeing null-pointer dereferenc in atmel-pcm.

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/ep93xx/ep93xx-pcm.c b/sound/soc/ep93xx/ep93xx-pcm.c
index a2de9c4..3fc9613 100644
--- a/sound/soc/ep93xx/ep93xx-pcm.c
+++ b/sound/soc/ep93xx/ep93xx-pcm.c
@@ -286,7 +286,6 @@
 static int ep93xx_pcm_new(struct snd_soc_pcm_runtime *rtd)
 {
 	struct snd_card *card = rtd->card->snd_card;
-	struct snd_soc_dai *dai = rtd->cpu_dai;
 	struct snd_pcm *pcm = rtd->pcm;
 	int ret = 0;
 
@@ -295,14 +294,14 @@
 	if (!card->dev->coherent_dma_mask)
 		card->dev->coherent_dma_mask = 0xffffffff;
 
-	if (dai->driver->playback.channels_min) {
+	if (pcm->streams[SNDRV_PCM_STREAM_PLAYBACK].substream) {
 		ret = ep93xx_pcm_preallocate_dma_buffer(pcm,
 					SNDRV_PCM_STREAM_PLAYBACK);
 		if (ret)
 			return ret;
 	}
 
-	if (dai->driver->capture.channels_min) {
+	if (pcm->streams[SNDRV_PCM_STREAM_CAPTURE].substream) {
 		ret = ep93xx_pcm_preallocate_dma_buffer(pcm,
 					SNDRV_PCM_STREAM_CAPTURE);
 		if (ret)