USB: fix usb serial pm counter decrement for disconnected interfaces

usb serial decrements the pm counter even if an interface has been
disconnected. If it was a logical disconnect the interface may belong
already to another driver. This patch introduces a check for disconnected
interfaces.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Cc: Stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -283,7 +283,10 @@ static void serial_close(struct tty_struct *tty, struct file * filp)
 	}
 
 	if (port->open_count == 0) {
+		mutex_lock(&port->serial->disc_mutex);
+		if (!port->serial->disconnected)
 			usb_autopm_put_interface(port->serial->interface);
+		mutex_unlock(&port->serial->disc_mutex);
 		module_put(port->serial->type->driver.owner);
 	}