e1000: Corrections for lem(4)/em(4) txcsum offload

Explicitly set ipcss/ipcse/ipcso for IPv6 per intel SDM as indicated in
inline comments.

Fix and consolidate 82543/82547 hwcsum exemption.

While here rearrange and expand some commentary.

(cherry picked from commit cbcab907f8ad1a4ac38dbc574c747ac2901faa54)