platform/x86: asus-wmi: Fix condition in charge_threshold_store()

This error handling is reversed so we return early.

Fixes: 84d8e80b

 ("platform/x86: asus-wmi: Refactor charge_threshold_store()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>