platform/x86: acer-wmi: Add ACER_CAP_SET_FUNCTION_MODE capability flag

Not all devices supporting WMID_GUID3 support the wmid3_set_function_mode()
call, leading to errors like these:

[   60.138358] acer_wmi: Enabling RF Button failed: 0x1 - 0xff
[   60.140036] acer_wmi: Enabling Launch Manager failed: 0x1 - 0xff

Add an ACER_CAP_SET_FUNCTION_MODE capability flag, so that these calls
can be disabled through the new force_caps mechanism.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20201019185628.264473-5-hdegoede@redhat.com

drivers/platform/x86/acer-wmi.c

@@ -210,6 +210,7 @@ struct hotkey_function_type_aa {
 #define ACER_CAP_BLUETOOTH		BIT(2)
 #define ACER_CAP_BRIGHTNESS		BIT(3)
 #define ACER_CAP_THREEG			BIT(4)
+#define ACER_CAP_SET_FUNCTION_MODE	BIT(5)
 
 /*
  * Interface type flags
@@ -2195,10 +2196,14 @@ static int __init acer_wmi_init(void)
 	if (acpi_video_get_backlight_type() != acpi_backlight_vendor)
 		interface->capability &= ~ACER_CAP_BRIGHTNESS;
 
+	if (wmi_has_guid(WMID_GUID3))
+		interface->capability |= ACER_CAP_SET_FUNCTION_MODE;
+
 	if (force_caps != -1)
 		interface->capability = force_caps;
 
-	if (wmi_has_guid(WMID_GUID3)) {
+	if (wmi_has_guid(WMID_GUID3) &&
+	    (interface->capability & ACER_CAP_SET_FUNCTION_MODE)) {
 		if (ACPI_FAILURE(acer_wmi_enable_rf_button()))
 			pr_warn("Cannot enable RF Button Driver\n");