[POWERPC] Make pci_bus_to_host()'s struct pci_bus * argument const

A) It's not modified and so it can be made const.  const is good.
B) If one has a function that was given a const pci_bus pointer and you
want to get a pointer to its pci_controller, you'll get a warning from gcc
when you use pci_bus_to_host().  This is the right way to stop that
warning.

Signed-off-by: Trent Piepho <tpiepho@freescale.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>