hwmon: (g762) handle cleanup with devm_add_action

Simplify code and use devm_add_action() to handle cleanup.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
[groeck: Dropped unnecessary dummy function and NULL check]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>