PCI/VPD: Add pci_vpd_find_id_string()

Add a pci_vpd_find_id_string() API function to retrieve the ID string from
VPD.

This way callers don't need pci_vpd_lrdt_size() any longer, and it can be
made private to the VPD core.

Link: https://lore.kernel.org/r/c5225bf6-8d29-970d-e271-0d7b52252630@gmail.com


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