ACPI / hotplug / PCI: Fix error code path in acpiphp_enumerate_slots()

One of the error code paths in acpiphp_enumerate_slots() is missing
a pci_dev_put(bridge->pci_dev) call, so add it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>