PCI: generic: Pass starting bus number to pci_scan_root_bus()

If the bus is being configured with a bus-range that does not start at
zero, pass that starting bus number to pci_scan_root_bus().  Passing the
incorrect value of zero causes attempted config accesses outside of the
supported range, which cascades to an OOPs spew and eventual kernel panic.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Will Deacon <will.deacon@arm.com>