hwmon: (lm80) Fix missing unlock on error in set_fan_div()

Add the missing unlock before return from function set_fan_div()
in the error handling case.

Fixes: c9c63915 ("hwmon: (lm80) fix a missing check of the status of SMBus read")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>