PCI/VPD: Allow access to valid parts of VPD if some is invalid

Previously, if we found any error in the VPD, we returned size 0, which
prevents access to all of VPD.  But there may be valid resources in VPD
before the error, and there's no reason to prevent access to those.

"off" covers only VPD resources known to have valid header tags.  In case
of error, return "off" (which may be zero if we haven't found any valid
header tags at all).

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>