hwmon: Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
./drivers/hwmon/hwmon.c:82:60-61: WARNING opportunity for kobj_to_dev()

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1614071667-5665-1-git-send-email-yang.lee@linux.alibaba.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>