Input: wm97xx-ts - fix exit path

Loading then unloading wm97xx-ts.ko when CONFIG_AC97_BUS=m
causes a WARNING: from drivers/base/driver.c:

Unexpected driver unregister!
WARNING: CPU: 0 PID: 1709 at ../drivers/base/driver.c:193 driver_unregister+0x30/0x40

Fix this by only calling driver_unregister() with the same
condition that driver_register() is called.

Fixes: ae9d1b5f ("Input: wm97xx: add new AC97 bus support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/wm97xx-core.c

@@ -929,7 +929,8 @@ static int __init wm97xx_init(void)
 
 static void __exit wm97xx_exit(void)
 {
-	driver_unregister(&wm97xx_driver);
+	if (IS_BUILTIN(CONFIG_AC97_BUS))
+		driver_unregister(&wm97xx_driver);
 	platform_driver_unregister(&wm97xx_mfd_driver);
 }