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>