iio: potentiometer: ds1803: Remove VLA usage

In preparation to enabling -Wvla, remove VLA usage and replace it
with fixed a fixed length array and therefore, prevent potential
stack overflow attacks.

Fixed as a part of the discussion to remove all VLAs from the kernel:
https://lkml.org/lkml/2018/3/7/621

Cc: keescook@chromium.org
Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/potentiometer/ds1803.c

@@ -64,7 +64,7 @@ static int ds1803_read_raw(struct iio_dev *indio_dev,
 	struct ds1803_data *data = iio_priv(indio_dev);
 	int pot = chan->channel;
 	int ret;
-	u8 result[indio_dev->num_channels];
+	u8 result[ARRAY_SIZE(ds1803_channels)];
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW: