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>

drivers/iio/light/lm3533-als.c

@@ -404,7 +404,7 @@ static int lm3533_als_get_hysteresis(struct iio_dev *indio_dev, unsigned nr,
 	return ret;
 }
 
-static int show_thresh_either_en(struct device *dev,
+static ssize_t show_thresh_either_en(struct device *dev,
 					struct device_attribute *attr,
 					char *buf)
 {
@@ -424,7 +424,7 @@ static int show_thresh_either_en(struct device *dev,
 	return scnprintf(buf, PAGE_SIZE, "%u\n", enable);
 }
 
-static int store_thresh_either_en(struct device *dev,
+static ssize_t store_thresh_either_en(struct device *dev,
 					struct device_attribute *attr,
 					const char *buf, size_t len)
 {