spi/s3c64xx: Fix compilation warning

Fix compilation warning by typecasting the tx_buf pointer.

[I'm not thrilled with resorting to a cast; but I cannot see a better
way to go about this.  I don't want to drop the const from struct
spi_transfer ~~glikely]
Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi_s3c64xx.c

@@ -508,8 +508,9 @@ static int s3c64xx_spi_map_mssg(struct s3c64xx_spi_driver_data *sdd,
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
 
 		if (xfer->tx_buf != NULL) {
-			xfer->tx_dma = dma_map_single(dev, xfer->tx_buf,
-						xfer->len, DMA_TO_DEVICE);
+			xfer->tx_dma = dma_map_single(dev,
+					(void *)xfer->tx_buf, xfer->len,
+					DMA_TO_DEVICE);
 			if (dma_mapping_error(dev, xfer->tx_dma)) {
 				dev_err(dev, "dma_map_single Tx failed\n");
 				xfer->tx_dma = XFER_DMAADDR_INVALID;