hwmon: (gpio-fan) Convert to use GPIO descriptors

This converts the GPIO fan driver to use GPIO descriptors. This way
we avoid indirection since the gpiolib anyway just use descriptors
inside, and we also get rid of explicit polarity handling: the
descriptors internally knows if the line is active high or active
low.

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