MIPS: pci-legacy: remove busn_resource field

No drivers set the busn_resource field in the pci_controller struct.
Commit 7ee214b5 ("MIPS: PCI: Remove unused busn_offset") almost
removed it over 3 years ago. Remove it for good to free up memory and
eliminate messages like:
  pci_bus 0000:00: root bus resource [??? 0x00000000 flags 0x0]
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/pci.h

@@ -38,7 +38,6 @@ struct pci_controller {
 	struct resource *io_resource;
 	unsigned long io_offset;
 	unsigned long io_map_base;
-	struct resource *busn_resource;
 
 #ifndef CONFIG_PCI_DOMAINS_GENERIC
 	unsigned int index;

arch/mips/pci/pci-legacy.c

@@ -89,7 +89,6 @@ static void pcibios_scanbus(struct pci_controller *hose)
 				hose->mem_resource, hose->mem_offset);
 	pci_add_resource_offset(&resources,
 				hose->io_resource, hose->io_offset);
-	pci_add_resource(&resources, hose->busn_resource);
 	list_splice_init(&resources, &bridge->windows);
 	bridge->dev.parent = NULL;
 	bridge->sysdata = hose;