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>