staging:iio:dac:ad5686: Check for negative values

Currently it is possible to write negative values to the ad5686's raw attribute.
This will cause undefined behaviour, so reject negative values.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Acked-by: NJonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/dac/ad5686.c

@@ -327,7 +327,7 @@ static int ad5686_write_raw(struct iio_dev *indio_dev,
 
 	switch (mask) {
 	case 0:
-		if (val > (1 << chan->scan_type.realbits))
+		if (val > (1 << chan->scan_type.realbits) || val < 0)
 			return -EINVAL;
 
 		mutex_lock(&indio_dev->mlock);