ixgb: revert an unwanted fix regarding tso/descriptors

There seemed to be another bug introduced as well as a performance hit
with the addtion of the sentinel descriptor workaround.  Removal of
this workaround appears to prevent the hang.  We'll take a risk
and remove it, as we had never seen the originally reported bug
under linux.

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