The intel-vbtn device will not be able to wake up the system any more
after removing the notify handler provided by its driver, so make
its sysfs attributes reflect that.

Fixes: 91f9e850 (platform: x86: intel-vbtn: Wake up the system from suspend-to-idle)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>