iio: sx9500: Add missing init in sx9500_buffer_pre{en,dis}able()

drivers/iio/proximity/sx9500.c: In function ‘sx9500_buffer_preenable’:
drivers/iio/proximity/sx9500.c:682: warning: ‘ret’ may be used uninitialized in this function
drivers/iio/proximity/sx9500.c: In function ‘sx9500_buffer_predisable’:
drivers/iio/proximity/sx9500.c:706: warning: ‘ret’ may be used uninitialized in this function

If active_scan_mask is empty, it will loop once more over all channels,
doing nothing.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Reviewed-by: Vlad Dogaru <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/proximity/sx9500.c

···
 static int sx9500_buffer_preenable(struct iio_dev *indio_dev)
 {
 	struct sx9500_data *data = iio_priv(indio_dev);
-	int ret, i;
+	int ret = 0, i;
 
 	mutex_lock(&data->mutex);
 
···
 static int sx9500_buffer_predisable(struct iio_dev *indio_dev)
 {
 	struct sx9500_data *data = iio_priv(indio_dev);
-	int ret, i;
+	int ret = 0, i;
 
 	iio_triggered_buffer_predisable(indio_dev);