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>