s390/pci: use pci_scan_root_bus

s390/pci: use pci_scan_root_bus

The pci config space accessors on s390 are (now) smart enough to
figure out if a pci function is available. So instead of calling
pci_create_root_bus and then pci_scan_single_device for each
available function just call pci_scan_root_bus and let the pci core
do the scanning (via config reads on all possible functions) and
device creation.

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>