hwmon: (gpio-fan) Use struct dev_pm_ops for power management

Make the gpio-fan driver define its PM callbacks through
a struct dev_pm_ops object rather than by using legacy PM hooks
in struct platform_driver.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
[linux@roeck-us.net: CONFIG_PM->CONFIG_PM_SLEEP, and remove unnecessary ()]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>