[S390] zcrypt: fix ap_device_list handling

In ap_device_probe() we can add the new ap device to the internal
device list only if the device probe function successfully returns.
Otherwise we might end up with an invalid device in the internal ap
device list.

Signed-off-by: Ralph Wuerthner <rwuerthn@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>