cxgb4: extend VPD parsing

Current code parses the VPD RO section for keywords but makes static
assumptions about the location of the section.  Remove them and parse
the VPD to find it.

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