leds: lp55xx: fix the sysfs read operation

According to a sysfs documentation(Documentation/filesystem/sysfs.txt),
scnprintf() should be used in a read operation method.
It guarantees safe buffer size(PAGE_SIZE) which is allocated by the sysfs.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

drivers/leds/leds-lp5521.c

@@ -360,7 +360,8 @@ static ssize_t lp5521_selftest(struct device *dev,
 	mutex_lock(&chip->lock);
 	ret = lp5521_run_selftest(chip, buf);
 	mutex_unlock(&chip->lock);
-	return sprintf(buf, "%s\n", ret ? "FAIL" : "OK");
+
+	return scnprintf(buf, PAGE_SIZE, "%s\n", ret ? "FAIL" : "OK");
 }
 
 /* device attributes */