da9030_battery: Don't use 0 as NULL

Noticed the new warning:

  CHECK   drivers/power/da9030_battery.c
drivers/power/da9030_battery.c:190:68: warning: Using plain integer as NULL pointer

This commit fixes the issue.
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

drivers/power/da9030_battery.c

@@ -187,8 +187,8 @@ static const struct file_operations bat_debug_fops = {
 
 static struct dentry *da9030_bat_create_debugfs(struct da9030_charger *charger)
 {
-	charger->debug_file = debugfs_create_file("charger", 0666, 0, charger,
-						 &bat_debug_fops);
+	charger->debug_file = debugfs_create_file("charger", 0666, NULL,
+						  charger, &bat_debug_fops);
 	return charger->debug_file;
 }