PCI: improve resource allocation under transparent bridges

We could run out of space under under 4g, but devices under transparent
bridges can use 64bit resources, so keep trying on the parent bus until
we hit a non-transparent bridge.

Impact: better support for assigning unassigned resources

Reviewed-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>