s390/ap: use the common driver-data pointer

The device struct provides a pointer for driver-private data. Use this
in the zcrypt drivers (as vfio_ap already does), and then remove the
custom pointer from the AP device structs.

As really_probe() will always clear the drvdata pointer on error, we
no longer have to do so ourselves.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>