cxgb4: collect serial config version from register

Collect serial config version information directly from an internal
register, instead of explicitly resizing VPD.

v2:
- Add comments on info stored in PCIE_STATIC_SPARE2 register.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>