spi: More sanity checks for transfers

Check that transfers are non-empty and that there is a completion for
them.

Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -1351,6 +1351,11 @@ static int __spi_async(struct spi_device *spi, struct spi_message *message)
 	struct spi_master *master = spi->master;
 	struct spi_transfer *xfer;
 
+	if (list_empty(&message->transfers))
+		return -EINVAL;
+	if (!message->complete)
+		return -EINVAL;
+
 	/* Half-duplex links include original MicroWire, and ones with
 	 * only one data pin like SPI_3WIRE (switches direction) or where
 	 * either MOSI or MISO is missing.  They can also be caused by