platform/x86: asus-wmi: cleanup AGFN fan handling

The asus-wmi driver currently uses the "AGFN" interface and
the FAN_CTRL device for fan control. According to the spec, this
interface is very dated and marked as pending removal from products
currently in development.

Clean up the way that the AGFN fan is detected and handled, also
preparing the driver for the introduction of an alternate fan
control method needed to support recent Asus products.

Not anticipating further development of this interface, simplify
the code by dropping any notion of being able to control multiple
AGFN fans (this was already limited to just a single fan through only
exposing a single fan in sysfs).

Check for the presence of AGFN fans at probe time, simplifying the code
flow in asus_hwmon_sysfs_is_visible().

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