spi: Ensure memory used for spi_write_then_read() is DMA safe

Use GFP_DMA in order to ensure that the memory we allocate for transfers
in spi_write_then_read() can be DMAed. On most platforms this will have
no effect.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi.c

@@ -1661,7 +1661,8 @@ int spi_write_then_read(struct spi_device *spi,
 	 * using the pre-allocated buffer or the transfer is too large.
 	 */
 	if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
-		local_buf = kmalloc(max((unsigned)SPI_BUFSIZ, n_tx + n_rx), GFP_KERNEL);
+		local_buf = kmalloc(max((unsigned)SPI_BUFSIZ, n_tx + n_rx),
+				    GFP_KERNEL | GFP_DMA);
 		if (!local_buf)
 			return -ENOMEM;
 	} else {