PCI/VPD: Remove struct pci_vpd.valid member

Instead of having a separate flag, use vp->len != 0 as indicator that VPD
validity has been checked.  Now vpd->len == PCI_VPD_SZ_INVALID indicates
that VPD is invalid.

Link: https://lore.kernel.org/r/9f777bc7-5316-e1b8-e5d4-f9f609bdb5dd@gmail.com
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>