hwmon: (mlxreg-fan) Use devm_thermal_of_cooling_device_register

Call devm_thermal_of_cooling_device_register() to register the cooling
device. Also introduce struct device *dev = &pdev->dev; to make the code
easier to read.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>