PCI: Fix devfn for VPD access through function 0

PCI: Fix devfn for VPD access through function 0

Commit 932c435caba8 ("PCI: Add dev_flags bit to access VPD through function
0") passes PCI_SLOT(devfn) for the devfn parameter of pci_get_slot().
Generally this works because we're fairly well guaranteed that a PCIe
device is at slot address 0, but for the general case, including
conventional PCI, it's incorrect.  We need to get the slot and then convert
it back into a devfn.

Fixes: 932c435caba8 ("PCI: Add dev_flags bit to access VPD through function 0")
Signed-off-by: Alex Williamson <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Acked-by: Myron Stowe <[email protected]>
Acked-by: Mark Rustad <[email protected]>
CC: [email protected]