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>
 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: