drivers/video/backlight/adp5520_bl.c: fix compiler warning in adp5520_show()

While compiling with make W=1 (gcc gcc (GCC) 4.7.2 20121109 (Red Hat
4.7.2-8)) the following warning happens:

  drivers/video/backlight/adp5520_bl.c: In function `adp5520_show':
  drivers/video/backlight/adp5520_bl.c:146:6: warning: variable `error' set but not used [-Wunused-but-set-variable]

fixed by checking the return value of the variable
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/backlight/adp5520_bl.c

@@ -143,13 +143,16 @@ static int adp5520_bl_setup(struct backlight_device *bl)
 static ssize_t adp5520_show(struct device *dev, char *buf, int reg)
 {
 	struct adp5520_bl *data = dev_get_drvdata(dev);
-	int error;
+	int ret;
 	uint8_t reg_val;
 
 	mutex_lock(&data->lock);
-	error = adp5520_read(data->master, reg, &reg_val);
+	ret = adp5520_read(data->master, reg, &reg_val);
 	mutex_unlock(&data->lock);
 
+	if (ret < 0)
+		return ret;
+
 	return sprintf(buf, "%u\n", reg_val);
 }