ASoC: Fix DMA channel leak in imx-pcm-dma-mx2 driver.

"snd_imx_pcm_hw_params" callback can be called
several times by the user (i.e. OSS emulation)
leading to a DMA channel leak.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/imx/imx-pcm-dma-mx2.c

@@ -88,11 +88,13 @@ static int imx_ssi_dma_alloc(struct snd_pcm_substream *substream,
 	iprtd->dma_data.dma_request = dma_params->dma;
 
 	/* Try to grab a DMA channel */
+	if (!iprtd->dma_chan) {
 		dma_cap_zero(mask);
 		dma_cap_set(DMA_SLAVE, mask);
 		iprtd->dma_chan = dma_request_channel(mask, filter, iprtd);
 		if (!iprtd->dma_chan)
 			return -EINVAL;
+	}
 
 	switch (params_format(params)) {
 	case SNDRV_PCM_FORMAT_S16_LE: