hwmon: (max6650) Fix lock imbalance

Add omitted update_lock to one switch/case in set_div.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>