PCI: pciehp: Prevent NULL dereference during probe

pciehp assumes that dev->subordinate, the struct pci_bus for a bridge's
secondary bus, exists.  But we do not create that bus if we run out of bus
numbers during enumeration.  This leads to a NULL dereference in
init_slot() (and other places).

Change pciehp_probe() to return -ENODEV when no secondary bus is present.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.2+