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

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