cxgb4: parse the VPD instead of relying on a static VPD layout

Some boards' VPDs contain additional keywords or have longer serial numbers,
meaning the keyword locations are variable.  Ditch the static layout and
use the pci_vpd_* family of functions to parse the VPD instead.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>