spi: Only call transfer_one() if we have buffers to transfer

Client drivers such as the ChomeOS EC driver sometimes use transfers with
no buffers and only a delay specified in order to allow a delay after the
assertion of /CS. Rather than require controller drivers handle this noop
case gracefully put checks in the core to ensure that we don't call into
the controller for such transfers.
Reported-by: Addy Ke <addy.ke@rock-chips.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -789,6 +789,7 @@ static int spi_transfer_one_message(struct spi_master *master,
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
 		trace_spi_transfer_start(msg, xfer);
 
+		if (xfer->tx_buf || xfer->rx_buf) {
 			reinit_completion(&master->xfer_completion);
 
 			ret = master->transfer_one(master, msg->spi, xfer);
@@ -808,9 +809,16 @@ static int spi_transfer_one_message(struct spi_master *master,
 			}
 
 			if (ms == 0) {
-			dev_err(&msg->spi->dev, "SPI transfer timed out\n");
+				dev_err(&msg->spi->dev,
+					"SPI transfer timed out\n");
 				msg->status = -ETIMEDOUT;
 			}
+		} else {
+			if (xfer->len)
+				dev_err(&msg->spi->dev,
+					"Bufferless transfer has length %u\n",
+					xfer->len);
+		}
 
 		trace_spi_transfer_stop(msg, xfer);