e1000: HWCSUM excemption fixes

Also disable IPV6 checksum offload.

Spell hw->mac.type < e1000_82543 as e1000_82542. Confusingly, chips
like 82540 and 82541 come later and do not have these issues. There
is no functional change here, as the enum was defined in such a way
it worked correctly. But this reads literally.

MFC after: 1 week