IGbE: Fix writeback conditions for i8254x GbE in updated data sheet.
IGbE: Fix writeback conditions for i8254x GbE in updated data sheet.

An older revision of the data sheet specified that txdctl.gran was 1 the granularity was
based on cache block and gran being 0 is based on descriptor count. The newer version of
the data sheet reverses this errata

pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,vovojh/gem5,hoangt/tpzsimul.gem5,vovojh/gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,vovojh/gem5,vovojh/gem5