s390/ap: wire up bus->probe and bus->remove

Hijacking the device_driver's probe/remove callbacks for purely
bus-internal logic is a very unconvential construct. Instead just set
up our callbacks in the AP bus struct, and really_probe() will call them
in the same way as before.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>