PCI/VPD: Add pci_vpd_check_csum()

VPD checksum information and checksum calculation are specified by PCIe
r5.0, sec 6.28.2.2.  Therefore checksum handling can and should be moved
into the PCI VPD core.

Add pci_vpd_check_csum() to validate the VPD checksum.

[bhelgaas: split to separate patch]
Link: https://lore.kernel.org/r/1643bd7a-088e-1028-c9b0-9d112cf48d63@gmail.com


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