pcibios_last_bus was apparently copied from x86.  On mn10300, it is
statically initialized to -1 and may be set with the "pci=lastbus=<X>"
boot option, but it is never tested.  This patch removes everything
related to pcibios_last_bus.
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>