PCI hotplug: make sure child bridges are enabled at hotplug time

Found one PCIe Module with several bridges built-in where a "cold"
hotadd doesn't work.

If we end up reassigning bridge windows at hotadd time, and have to loop
through assigning new ranges, we won't end up enabling the child bridges
because the first assignment pass already tried to enable them, which
prevents __pci_bridge_assign_resource from updating the windows.

So try to move enabling of child bridges to the end, and only do it
once.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>