asus-laptop: correct error handling in asus_read_brightness()

It is possible that acpi_evaluate_integer might fail and value would not be
set to any value so correct this defect by returning 0 in case of an
error. This is also the correct thing to return because the backlight
subsystem will print the old value of brightness in this case.
Signed-off-by: NGiedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: NDarren Hart <dvhart@linux.intel.com>

drivers/platform/x86/asus-laptop.c

@@ -775,8 +775,10 @@ static int asus_read_brightness(struct backlight_device *bd)
 
 	rv = acpi_evaluate_integer(asus->handle, METHOD_BRIGHTNESS_GET,
 				   NULL, &value);
-	if (ACPI_FAILURE(rv))
+	if (ACPI_FAILURE(rv)) {
 		pr_warn("Error reading brightness\n");
+		return 0;
+	}
 
 	return value;
 }