staging: iio: isl29018: fix sparse warning regarding incorrect type (different signedness)

Fix the following sparse warning:

drivers/staging/iio/light/isl29018.c:508:50: warning: incorrect type in
argument 3 (different signedness)
drivers/staging/iio/light/isl29018.c:508:50:    expected unsigned int
*conf_adc_bit
drivers/staging/iio/light/isl29018.c:508:50:    got int *<noident>
Signed-off-by: Teodora Baluta <teodora.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/light/isl29018.c

@@ -454,7 +454,7 @@ static const struct attribute_group isl29108_group = {
 static int isl29018_chip_init(struct isl29018_chip *chip)
 {
 	int status;
-	int new_adc_bit;
+	unsigned int new_adc_bit;
 	unsigned int new_range;
 
 	/* Code added per Intersil Application Note 1534: