[S390] qdio: fix broken pointer in case of CONFIG_DEBUG_FS is disabled

If CONFIG_DEBUG_FS is disabled debugfs_create_file returns -ENODEV instead
of zero. Since the return value is stored and used at shutdown to remove
existing entries an OOPS may be triggered.

Add a check of the debugfs_create_file return value and in case of an error
set the entry to NULL so it will be ignored at shutdown.

Signed-off-by: Jan Glauber <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

drivers/s390/cio/qdio_debug.c

···
 		 q->nr);
 	debugfs_queues[i] = debugfs_create_file(name, S_IFREG | S_IRUGO | S_IWUSR,
 						debugfs_root, q, &debugfs_fops);
+	if (IS_ERR(debugfs_queues[i]))
+		debugfs_queues[i] = NULL;
 }
 
 void qdio_setup_debug_entries(struct qdio_irq *irq_ptr, struct ccw_device *cdev)