MIPS: PCI: Remove unused busn_offset

pci_add_resource_offset() is for host bridge windows where the bridge
translates CPU addresses to PCI bus addresses by adding an offset.  To my
knowledge, no host bridge translates bus numbers, so this is only useful
for MEM and IO windows.  In any event, host->busn_offset is never set to
anything other than zero, so pci_add_resource() is sufficient.

a2e50f53 ("MIPS: PCI: Add a hook for IORESOURCE_BUS in
pci_controller/bridge_controller") also added busn_resource itself.  This
is currently unused but may be used by future SGI IP27 fixes, so I left it
there.

Tested-by: Joshua Kinard <kumba@gentoo.org>	# SGI IP30 and IP27
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Joshua Kinard <kumba@gentoo.org>