HID: asus: fix build warning wiht CONFIG_ASUS_WMI disabled

asus_wmi_evaluate_method() is an empty dummy function when CONFIG_ASUS_WMI
is disabled, or not reachable from a built-in device driver. This leads to
a theoretical evaluation of an uninitialized variable that the compiler
complains about, failing to check that the hardcoded return value makes
this an unreachable code path:

In file included from include/linux/printk.h:336,
                 from include/linux/kernel.h:14,
                 from include/linux/list.h:9,
                 from include/linux/dmi.h:5,
                 from drivers/hid/hid-asus.c:29:
drivers/hid/hid-asus.c: In function 'asus_input_configured':
include/linux/dynamic_debug.h:135:3: error: 'value' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   __dynamic_dev_dbg(&descriptor, dev, fmt, \
   ^~~~~~~~~~~~~~~~~
drivers/hid/hid-asus.c:359:6: note: 'value' was declared here
  u32 value;
      ^~~~~

With an extra IS_ENABLED() check, the warning goes away.

Fixes: 3b692c55 ("HID: asus: only support backlight when it's not driven by WMI")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-asus.c

@@ -359,6 +359,9 @@ static bool asus_kbd_wmi_led_control_present(struct hid_device *hdev)
 	u32 value;
 	int ret;
 
+	if (!IS_ENABLED(CONFIG_ASUS_WMI))
+		return false;
+
 	ret = asus_wmi_evaluate_method(ASUS_WMI_METHODID_DSTS2,
 				       ASUS_WMI_DEVID_KBD_BACKLIGHT, 0, &value);
 	hid_dbg(hdev, "WMI backlight check: rc %d value %x", ret, value);