dmaengine: add helper function for slave_single

For clients which require a single slave transfer and dont want to be bothered
about the scatterlist api, this helper gives simple API for this transfer and
creates single scatterlist for DMA API

Idea from Russell King

Signed-off-by: Vinod Koul <vinod.koul@intel.com>

include/linux/dmaengine.h

@@ -24,6 +24,7 @@
 #include <linux/device.h>
 #include <linux/uio.h>
 #include <linux/dma-direction.h>
+#include <linux/scatterlist.h>
 
 struct scatterlist;
 
@@ -519,6 +520,16 @@ static inline int dmaengine_slave_config(struct dma_chan *chan,
 			(unsigned long)config);
 }
 
+static inline struct dma_async_tx_descriptor *dmaengine_prep_slave_single(
+	struct dma_chan *chan, void *buf, size_t len,
+	enum dma_data_direction dir, unsigned long flags)
+{
+	struct scatterlist sg;
+	sg_init_one(&sg, buf, len);
+
+	return chan->device->device_prep_slave_sg(chan, &sg, 1, dir, flags);
+}
+
 static inline int dmaengine_terminate_all(struct dma_chan *chan)
 {
 	return dmaengine_device_control(chan, DMA_TERMINATE_ALL, 0);