spi: uniphier: fix zero-length transfer

The zero-length transfer results in timeout error because
the transfer doesn't start.
This commit modified to return success in this case.
Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-uniphier.c

@@ -331,6 +331,10 @@ static int uniphier_spi_transfer_one(struct spi_master *master,
 	struct device *dev = master->dev.parent;
 	unsigned long time_left;
 
+	/* Terminate and return success for 0 byte length transfer */
+	if (!t->len)
+		return 0;
+
 	uniphier_spi_setup_transfer(spi, t);
 
 	reinit_completion(&priv->xfer_done);