hwmon: (max6650) Switch to using the new API kobj_to_dev()

Switch to using the new API kobj_to_dev() to fix the below warnning:
drivers/hwmon/max6650.c:324:60-61: WARNING opportunity for
kobj_to_dev().

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1609376621-46463-1-git-send-email-tiantao6@hisilicon.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>