hwmon: (gpio-fan) Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
drivers/hwmon/gpio-fan.c:302:60-61: WARNING opportunity for
kobj_to_dev()

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1609211837-54930-1-git-send-email-tiantao6@hisilicon.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>