pci: Add VPD information field helper functions

This patch adds a preprocessor constant to describe the PCI VPD
information field header size and an inline function to extract the
size of the information field itself.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: David S. Miller <davem@davemloft.net>