cxgb4: Search VPD with pci_vpd_find_ro_info_keyword()

Use pci_vpd_find_ro_info_keyword() to search for keywords in VPD to
simplify the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>