atmel_spi: allow transfer when max_speed_hz = 0

For some reason I have to slowdown clock to touchscreen device.

In atmel_spi_setup() there is comment that max_speed_hz == 0 means as slow
as possible and divider is set to maximum value.  But in
atmel_spi_transfer() function is check against not zero max_speed_hz with
EINVAL returned.

Probably driver should setup divider for each transfer based on
transfer->speed_hz value, but I think that would be not necessary overhead
as all used devices have constant clock.

Below patch works fine for me.
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/atmel_spi.c

@@ -670,8 +670,7 @@ static int atmel_spi_transfer(struct spi_device *spi, struct spi_message *msg)
 	dev_dbg(controller, "new message %p submitted for %s\n",
 			msg, spi->dev.bus_id);
 
-	if (unlikely(list_empty(&msg->transfers)
-			|| !spi->max_speed_hz))
+	if (unlikely(list_empty(&msg->transfers)))
 		return -EINVAL;
 
 	if (as->stopping)