PCI: add pci_bus_for_each_resource(), remove direct bus->resource[] refs

No functional change; this converts loops that iterate from 0 to
PCI_BUS_NUM_RESOURCES through pci_bus resource[] table to use the
pci_bus_for_each_resource() iterator instead.

This doesn't change the way resources are stored; it merely removes
dependencies on the fact that they're in a table.

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