platform/x86: asus-wmi: fix CPU fan control on recent products

Previously, asus-wmi was using the AGFN interface and FAN_CTRL device
for CPU fan control. However, this code has been found to be not fully
working on some recent products, and having checked the spec, these
interfaces are marked as being removed from future products currently
in development.

The replacement appears to be the CPU_FAN device, added in spec version
8.3 (March 2014) and present on many modern Asus laptops.

Add support for this device, and use it whenever it is detected.
The older approach based on AGFN and FAN_CTRL is used as a fallback
on products that do not have such device.

Other than switching between automatic and full speed, there is
no fan speed control through this new interface.

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