cxgb4: Allow for backward compatibility with new VPD scheme.

New scheme calls for 3rd party VPD at offset 0x0 and Chelsio VPD at offset
0x400 of the function.  If no 3rd party VPD is present, then a copy of
Chelsio's VPD will be at offset 0x0 to keep in line with PCI spec which
requires the VPD to be present at offset 0x0.

Signed-off-by: Santosh Rastapur <santosh@chelsio.com>
Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>