ixgb: Fix early TSO completion

This fix was already merged in commit 96f9c2e2
but reverted in commit 989316dd. After
stresstesting we found that the fix does not add new regressions and
works around a TX hang spotted by several users.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>