spi: Do not require a completion

There is no real reason why we require transfers to have a completion and
the only user of the completion now checks to see if one has been provided
before using it so stop enforcing this. This makes it more convenient for
drivers to chain multiple asynchronous transfers together.
Signed-off-by: NMark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -1621,8 +1621,6 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 
 	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