USB: atm/cxacru, fix lock imbalance

We do not hold mutex in one place in cxacru_cm, but unlock it on fail path.
Fix this.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Simon Arlott <cxacru@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>