PCI: portdrv: Restore PCI config state on slot reset

The port's config space may be cleared after a link reset, which wipes out
the bridge's bus and memory windows.  Restore the config space that was
saved during probe so we can access downstream devices.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>