iio: lm3533-als: Fix build warnings

Fix below build warnings:
  CC [M]  drivers/iio/light/lm3533-als.o
drivers/iio/light/lm3533-als.c:667:8: warning: initialization from incompatible pointer type [enabled by default]
drivers/iio/light/lm3533-als.c:667:8: warning: (near initialization for 'dev_attr_in_illuminance0_thresh_either_en.show') [enabled by default]
drivers/iio/light/lm3533-als.c:667:8: warning: initialization from incompatible pointer type [enabled by default]
drivers/iio/light/lm3533-als.c:667:8: warning: (near initialization for 'dev_attr_in_illuminance0_thresh_either_en.store') [enabled by default]

Signed-off-by: Axel Lin <axel.lin@gmail.com>