iio: light: signedness bug in stk3310_write_raw()

"index" needs to be signed for the error handling to work.

Fixes: be9e6229 ('iio: light: Add support for Sensortek STK3310')
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/light/stk3310.c

@@ -370,7 +370,7 @@ static int stk3310_write_raw(struct iio_dev *indio_dev,
 			     int val, int val2, long mask)
 {
 	int ret;
-	unsigned int index;
+	int index;
 	struct stk3310_data *data = iio_priv(indio_dev);
 
 	switch (mask) {