hwmon: (max6650) Use devm function to register thermal device

Use devm_thermal_of_cooling_device_register to register the thermal
cooling device. This lets us drop the remove function.

At the same time, use 'dev' variable in probe function consistently.

Cc: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>