s390/pci: reenable per default

HW, FW and Linux support is in a better shape now - let's reenable
pci bus probing per default.

Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>