s390/vfio-ap: clean up vfio_ap_drv's definition

Define & initialize the driver struct in one go, so that everything
is in one place.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>