PCI: fix bridge I/O window bus-to-resource conversion

In 5bfa14ed9f3c, I forgot to initialize res2.flags before calling
pcibios_bus_to_resource(), which depends on the resource type to locate the
correct aperture.  This bug won't hurt x86, which currently never has an
offset between bus and CPU addresses, but will affect other architectures.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>