hwmon: (gpio-fan) Send around device pointer

The driver is storing the struct platform_device *pdev pointer
but what it is really using and want to pass around is a
struct device *dev pointer.

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