platform/x86: thinkpad_acpi: Don't register keyboard_lang unnecessarily

All recent ThinkPad BIOS-es support the GSKL method used to query the
keyboard-layout used by the ECFW for the SHIFT + other-key key-press
emulation for special keys such as e.g. the '=', '(' and ')' keys
above the numpad on 15" models.

So just checking for the method is not a good indicator of the
model supporting getting/setting the keyboard_lang.

On models where this is not supported GSKL succeeds, but it returns
METHOD_ERR in the returned integer to indicate that this is not
supported on this model.

Add a check for METHOD_ERR and return -ENODEV if it is set to
avoid registering a non-working keyboard_lang sysfs-attr on models
where this is not supported.

Cc: Nitin Joshi <njoshi1@lenovo.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210125205258.135664-1-hdegoede@redhat.com

drivers/platform/x86/thinkpad_acpi.c

@@ -10047,6 +10047,13 @@ static int get_keyboard_lang(int *output)
 	if (!acpi_evalf(gskl_handle, &kbd_lang, NULL, "dd", 0x02000000))
 		return -EIO;
 
+	/*
+	 * METHOD_ERR gets returned on devices where there are no special (e.g. '=',
+	 * '(' and ')') keys which use layout dependent key-press emulation.
+	 */
+	if (kbd_lang & METHOD_ERR)
+		return -ENODEV;
+
 	*output = kbd_lang;
 
 	return 0;