hwmon: (npcm750-pwm-fan) Use devm_thermal_of_cooling_device_register

Use devm_thermal_of_cooling_device_register() to register the cooling
device. As a side effect, this fixes a driver bug:
thermal_cooling_device_unregister() was not called on device removal.

Fixes: f1fd4a4d ("hwmon: Add NPCM7xx PWM and Fan driver")
Cc: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>