PCI: Increment max correctly in pci_scan_bridge()

This patch fixes two small issues:
 - If pci_add_new_bus() fails, max must not be incremented. Otherwise
   an incorrect value is returned from pci_scan_bridge().
 - If the bus is already present, max must be incremented. I think
   that this case should only be hit if we trigger a manual rescan of a
   CardBus bridge.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>