s390/ap: use the common device_driver pointer

The device struct itself already contains a pointer to its driver.
Use this consistently, instead of duplicating it.

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>