platform-drivers-x86: asus-wmi: fix resource leak in asus_wmi_led_exit

This patch fixes wrong test for successful registered led_classdev,
and also fixes a leak due to missing led_classdev_unregister call
for asus->kbd_led in asus_wmi_led_exit.

device_create returns &struct device pointer on success, or ERR_PTR() on error.
Thus if led_classdev_register fails, led_cdev->dev is always not NULL.

Thus to unregister a successful registered led_classdev, we should check
(!IS_ERR_OR_NULL(asus->tpd_led.dev)) instead of (asus->tpd_led.dev).

we use IS_ERR_OR_NULL instead of IS_ERR because if we havn't call
led_classdev_register, the led_cdev->dev is NULL.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>