asus-wmi: Convert to use devm_hwmon_device_register_with_groups

Simplify the code and avoid race conditions due to late sysfs attribute
registration. Also replace SENSOR_DEVICE_ATTR with DEVICE_ATTR;
the additional parameter is not used and thus unnecessary.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>