PCI: mips: use generic INTx swizzle from PCI core

Use the generic pci_common_swizzle() instead of arch-specific code.

Note that pci_common_swizzle() loops based on dev->bus->self, not
dev->bus->parent as the mips common_swizzle() did.  I think they
are equivalent for this purpose.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>