iio: light: tsl2591: fix some signedness bugs

These variables need to be int for the error handling to work.

Fixes: 2335f0d7 ("iio: light: Added AMS tsl2591 driver implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YJ52r1XZ44myD9Xx@mwandaSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/light/tsl2591.c

@@ -213,7 +213,7 @@ static int tsl2591_gain_to_multiplier(const u8 als_gain)
 	}
 }
 
-static u8 tsl2591_multiplier_to_gain(const u32 multiplier)
+static int tsl2591_multiplier_to_gain(const u32 multiplier)
 {
 	switch (multiplier) {
 	case TSL2591_CTRL_ALS_LOW_GAIN_MULTIPLIER:
@@ -783,8 +783,8 @@ static int tsl2591_write_raw(struct iio_dev *indio_dev,
 			     int val, int val2, long mask)
 {
 	struct tsl2591_chip *chip = iio_priv(indio_dev);
-	u32 int_time;
-	u8 gain;
+	int int_time;
+	int gain;
 	int ret;
 
 	mutex_lock(&chip->als_mutex);