USB: image/mdc800, fix lock imbalance

There is an omitted unlock in mdc800_usb_probe's fail path. Add it.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Henning Zabel <henning@uni-paderborn.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/image/mdc800.c

@@ -499,6 +499,7 @@ static int mdc800_usb_probe (struct usb_interface *intf,
 	retval = usb_register_dev(intf, &mdc800_class);
 	if (retval) {
 		dev_err(&intf->dev, "Not able to get a minor for this device.\n");
+		mutex_unlock(&mdc800->io_lock);
 		return -ENODEV;
 	}