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>