s390/pci: cleanup hotplug code

Provide wrappers for the [de]configure operations, add some error
handling, and use pci_scan_slot instead of pci_scan_single_device.

Reviewed-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>