e1000e: always set transmit descriptor control registers the same

The hardware erratum workaround where the TXDCTL register must be the same
setting for both queues should always be done.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>