PCI: do not enable bridges more than once

In preparation for PCI core hotplug, we need to ensure that we do
not attempt to re-enable bridges that have already been enabled.

Reported-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>