Input: wistron - fix test for CONFIG_PM

This should fix the following compile warning:

drivers/input/misc/wistron_btns.c:1331:5: warning: "CONFIG_PM" is not defined
Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/misc/wistron_btns.c

@@ -1328,7 +1328,7 @@ static struct platform_driver wistron_driver = {
 	.driver		= {
 		.name	= "wistron-bios",
 		.owner	= THIS_MODULE,
-#if CONFIG_PM
+#ifdef CONFIG_PM
 		.pm	= &wistron_pm_ops,
 #endif
 	},