PCI: connect struct pci_dev to struct pci_slot

The introduction of struct pci_slot (f46753c5

)
added a struct pci_slot pointer to struct pci_dev, but we forgot to
associate the two.

Connect the two structs together; the interesting portions of the object
lifetimes are:

	- when a new pci_slot is created, connect it to the appropriate
	  pci_dev's. A single pci_slot may be associated with multiple
	  pci_dev's, e.g. any multi-function PCI device.

	- when a pci_slot is released, look for all the pci_dev's it was
	  associated with, and set their pci_slot pointers to NULL

	- when a pci_dev is created, look for slots to associate with.

Note -- when a pci_dev is released, we don't need to do any bookkeeping,
since pci_slot's do not have pointers to pci_dev's.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>