hwmon: (w83791d) fan 4/5 pins can also be used for gpio

Pins fan/pwm 4-5 can be in use as GPIO. If that is the case, do not
create their sysfs-interface. 

Signed-off-by: Marc Hulsman <m.hulsman@tudelft.nl>
Acked-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>