dma: imx-sdma: buf_tail should be initialize in prepare function

This fix audio underrun issue. When SNDRV_PCM_TRIGGER_STOP
and SNDRV_PCM_TRIGGER_START, it calls prepare again. buf_tail
should be reset to zero.
So move buf_tail initialization into prepare function.
Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/imx-sdma.c

@@ -815,8 +815,6 @@ static int sdma_request_channel(struct sdma_channel *sdmac)
 
 	init_completion(&sdmac->done);
 
-	sdmac->buf_tail = 0;
-
 	return 0;
 out:
 
@@ -927,6 +925,8 @@ static struct dma_async_tx_descriptor *sdma_prep_slave_sg(
 
 	sdmac->flags = 0;
 
+	sdmac->buf_tail = 0;
+
 	dev_dbg(sdma->dev, "setting up %d entries for channel %d.\n",
 			sg_len, channel);
 
@@ -1027,6 +1027,8 @@ static struct dma_async_tx_descriptor *sdma_prep_dma_cyclic(
 
 	sdmac->status = DMA_IN_PROGRESS;
 
+	sdmac->buf_tail = 0;
+
 	sdmac->flags |= IMX_DMA_SG_LOOP;
 	sdmac->direction = direction;
 	ret = sdma_load_context(sdmac);