backlight: adp88x0: Fix uninitialized variable use

gcc correctly warns about both the adp8860 and adp8870 backlight
drivers using an uninitialized variable in their error handling
path:

drivers/video/backlight/adp8870_bl.c: In function 'adp8870_bl_ambient_light_zone_store':
drivers/video/backlight/adp8870_bl.c:811:11: warning: 'reg_val' may be used uninitialized in this function

This changes the code to only write back the data if it was
correctly read to start with.

As a side-note, the drivers are mostly identical, so I think they
should really be merged into one file to avoid having to fix every
bug twice.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/adp8860_bl.c

@@ -621,10 +621,12 @@ static ssize_t adp8860_bl_ambient_light_zone_store(struct device *dev,
 
 		/* Set user supplied ambient light zone */
 		mutex_lock(&data->lock);
-		adp8860_read(data->client, ADP8860_CFGR, &reg_val);
+		ret = adp8860_read(data->client, ADP8860_CFGR, &reg_val);
+		if (!ret) {
 			reg_val &= ~(CFGR_BLV_MASK << CFGR_BLV_SHIFT);
 			reg_val |= (val - 1) << CFGR_BLV_SHIFT;
 			adp8860_write(data->client, ADP8860_CFGR, reg_val);
+		}
 		mutex_unlock(&data->lock);
 	}
 

drivers/video/backlight/adp8870_bl.c

@@ -807,10 +807,12 @@ static ssize_t adp8870_bl_ambient_light_zone_store(struct device *dev,
 
 		/* Set user supplied ambient light zone */
 		mutex_lock(&data->lock);
-		adp8870_read(data->client, ADP8870_CFGR, &reg_val);
+		ret = adp8870_read(data->client, ADP8870_CFGR, &reg_val);
+		if (!ret) {
 			reg_val &= ~(CFGR_BLV_MASK << CFGR_BLV_SHIFT);
 			reg_val |= (val - 1) << CFGR_BLV_SHIFT;
 			adp8870_write(data->client, ADP8870_CFGR, reg_val);
+		}
 		mutex_unlock(&data->lock);
 	}