s390/pci: fix removal of nonexistent pci bus

If we remove a pci bus after receiving a hotplug notification we need
to check if the bus is actually present (creation of the pci bus
during an earlier notification may have been failed).

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>