gpio: max7301: fix driver for use with CONFIG_VMAP_STACK

spi_read() and spi_write() require DMA-safe memory. When
CONFIG_VMAP_STACK is selected, those functions cannot be used
with buffers on stack.

This patch replaces calls to spi_read() and spi_write() by
spi_write_then_read() which doesn't require DMA-safe buffers.

Fixes: 0c36ec31 ("gpio: gpio driver for max7301 SPI GPIO expander")
Cc: <stable@vger.kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-max7301.c

@@ -25,7 +25,7 @@ static int max7301_spi_write(struct device *dev, unsigned int reg,
 	struct spi_device *spi = to_spi_device(dev);
 	u16 word = ((reg & 0x7F) << 8) | (val & 0xFF);
 
-	return spi_write(spi, (const u8 *)&word, sizeof(word));
+	return spi_write_then_read(spi, &word, sizeof(word), NULL, 0);
 }
 
 /* A read from the MAX7301 means two transfers; here, one message each */
@@ -37,14 +37,8 @@ static int max7301_spi_read(struct device *dev, unsigned int reg)
 	struct spi_device *spi = to_spi_device(dev);
 
 	word = 0x8000 | (reg << 8);
-	ret = spi_write(spi, (const u8 *)&word, sizeof(word));
-	if (ret)
-		return ret;
-	/*
-	 * This relies on the fact, that a transfer with NULL tx_buf shifts out
-	 * zero bytes (=NOOP for MAX7301)
-	 */
-	ret = spi_read(spi, (u8 *)&word, sizeof(word));
+	ret = spi_write_then_read(spi, &word, sizeof(word), &word,
+				  sizeof(word));
 	if (ret)
 		return ret;
 	return word & 0xff;