atmel_spi: fix warning In function 'atmel_spi_dma_map_xfer'

passing argument 2 of 'dma_map_single' discards qualifiers from pointer target type
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

drivers/spi/atmel_spi.c

@@ -352,8 +352,12 @@ atmel_spi_dma_map_xfer(struct atmel_spi *as, struct spi_transfer *xfer)
 
 	xfer->tx_dma = xfer->rx_dma = INVALID_DMA_ADDRESS;
 	if (xfer->tx_buf) {
+		/* tx_buf is a const void* where we need a void * for the dma
+		 * mapping */
+		void *nonconst_tx = (void *)xfer->tx_buf;
+
 		xfer->tx_dma = dma_map_single(dev,
-				(void *) xfer->tx_buf, xfer->len,
+				nonconst_tx, xfer->len,
 				DMA_TO_DEVICE);
 		if (dma_mapping_error(dev, xfer->tx_dma))
 			return -ENOMEM;