s390/pci/hotplug: convert to be builtin only

Convert s390' pci hotplug to be builtin only, with no module option.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>