PCI: Drop temporary variable in pci_assign_unassigned_resources()

Drop the "bus" temporary variable.  No functional change, but simplifies
later patch slightly.

[bhelgaas: changelog, make same change in
pci_assign_unassigned_bridge_resources() to keep it parallel with
pci_assign_unassigned_resources()]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>