platform/x86: asus-wmi: Refactor charge_threshold_store()

There are few issues with the current code:
 - the error code from kstrtouint() is shadowed
 - the error code from asus_wmi_set_devstate() is ignored
 - the extra check against 0 for count (this is guaranteed by sysfs)

Fix these issues by doing a slight refactoring of charge_threshold_store().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>