USB: wusbcore/wa-xfer, fix lock imbalance

Fix locking on one wa_urb_enqueue_b's fail path. There was omitted unlock.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>