platform/x86: asus-wmi: fix cpufv sysfs file permission

The cpufv sysfs file is defined as readable by anyone even if the
attribute does not have a show function. The result of every read is an
IO error. This file should be write only.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>