spi: core: make zero length transfer valid again

Zero length transfer becomes invalid since
"spi: core: Validate length of the transfers in message" commit,
but it should be valid to support an odd device, for example, which
requires long delay between chipselect and the first transfer, etc.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -1617,7 +1617,7 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 {
 	struct spi_master *master = spi->master;
 	struct spi_transfer *xfer;
-	int w_size, n_words;
+	int w_size;
 
 	if (list_empty(&message->transfers))
 		return -EINVAL;
@@ -1680,9 +1680,8 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 		else
 			w_size = 4;
 
-		n_words = xfer->len / w_size;
 		/* No partial transfers accepted */
-		if (!n_words || xfer->len % w_size)
+		if (xfer->len % w_size)
 			return -EINVAL;
 
 		if (xfer->speed_hz && master->min_speed_hz &&