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>