hwmon: (gpio-fan) Add missing mutex locks

set_fan_speed() is expected to be called with fan_data->lock being locked.
Add locking for proper synchronization.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20250210145934.761280-3-alexander.stein@ew.tq-group.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>