PCI/VPD: Treat initial 0xff as missing EEPROM

Previously we assumed that the first tag being 0x00 meant an EEPROM was
missing.  The first tag being 0xff means the same thing; check for that
also.

[bhelgaas: rework error mesage]
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>