PCI: Add pci_dev_id() helper

In several places in the kernel we find PCI_DEVID used like this:

  PCI_DEVID(dev->bus->number, dev->devfn)

Add a "pci_dev_id(struct pci_dev *dev)" helper to simplify callers.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>