[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>

include/asm-powerpc/pci-bridge.h

@@ -117,7 +117,7 @@ struct pci_controller {
 
 #ifndef CONFIG_PPC64
 
-static inline struct pci_controller *pci_bus_to_host(struct pci_bus *bus)
+static inline struct pci_controller *pci_bus_to_host(const struct pci_bus *bus)
 {
 	return bus->sysdata;
 }
@@ -235,7 +235,7 @@ extern void pcibios_fixup_new_pci_devices(struct pci_bus *bus);
 
 extern int pcibios_remove_root_bus(struct pci_controller *phb);
 
-static inline struct pci_controller *pci_bus_to_host(struct pci_bus *bus)
+static inline struct pci_controller *pci_bus_to_host(const struct pci_bus *bus)
 {
 	struct device_node *busdn = bus->sysdata;