hwmon: (gpio-fan) Mandate OF_GPIO and cut pdata path

We have no users of platform data, we made platform data driver-local,
so cut all #ifdefs for the platform data case, and depend on the
Kconfig CONFIG_OF_GPIO symbol.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>