PCI: split up pci_read_bridge_bases()

No functional change; this breaks up pci_read_bridge_bases() into separate
pieces for the I/O, memory, and prefetchable memory windows, similar to how
Yinghai recently split up pci_setup_bridge() in 68e84ff3bdc.

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