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>

include/linux/platform_data/x86/asus-wmi.h

@@ -12,7 +12,7 @@
 #define ASUS_WMI_METHODID_GPID		0x44495047 /* Get Panel ID?? (Resol) */
 #define ASUS_WMI_METHODID_QMOD		0x444F4D51 /* Quiet MODe */
 #define ASUS_WMI_METHODID_SPLV		0x4C425053 /* Set Panel Light Value */
-#define ASUS_WMI_METHODID_AGFN		0x4E464741 /* FaN? */
+#define ASUS_WMI_METHODID_AGFN		0x4E464741 /* Atk Generic FuNction */
 #define ASUS_WMI_METHODID_SFUN		0x4E554653 /* FUNCtionalities */
 #define ASUS_WMI_METHODID_SDSP		0x50534453 /* Set DiSPlay output */
 #define ASUS_WMI_METHODID_GDSP		0x50534447 /* Get DiSPlay output */
@@ -72,7 +72,7 @@
 
 /* Fan, Thermal */
 #define ASUS_WMI_DEVID_THERMAL_CTRL	0x00110011
-#define ASUS_WMI_DEVID_FAN_CTRL		0x00110012
+#define ASUS_WMI_DEVID_FAN_CTRL		0x00110012 /* deprecated */
 
 /* Power */
 #define ASUS_WMI_DEVID_PROCESSOR_STATE	0x00120012