PCI/VPD: Add helper pci_get_func0_dev()

Factor out the "get function 0" logic into pci_get_func0_dev().

[bhelgaas: keep PCI_DEVFN(PCI_SLOT()) instead of exposing implementation
details, commit log]
Link: https://lore.kernel.org/r/75d1f619-8a35-690d-8fc8-e851264a4bbb@gmail.com


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