platform/x86: asus-wmi: Fix hwmon device cleanup

The driver does not clean up the hwmon device on exit or error. To
reproduce the bug, repeat rmmod, insmod to verify that device number
/sys/devices/platform/asus-nb-wmi/hwmon/hwmon?? grows every time. Replace
call for registering device with devm_* version that unregisters it
automatically.

Signed-off-by: Yurii Pavlovskyi <yurii.pavlovskyi@gmail.com>
Reviewed-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>