spi: core: Fix transfer failure when master->transfer_one returns positive value

master->transfer_one returns positive value is not a error.
So set ret to 0 when master->transfer_one returns positive value.
Otherwise, I hit "spi_master spi0: failed to transfer one message from queue"
error when my transfer_one callback returns 1.
Signed-off-by: NAxel Lin <axel.lin@ingics.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -599,8 +599,10 @@ static int spi_transfer_one_message(struct spi_master *master,
 			goto out;
 		}
 
-		if (ret > 0)
+		if (ret > 0) {
+			ret = 0;
 			wait_for_completion(&master->xfer_completion);
+		}
 
 		trace_spi_transfer_stop(msg, xfer);