i40e: clear qtx_head before enabling Tx queue

Make sure the "new" qtx_head[q] register is cleared before
enabling the Tx queue.

Change-ID: I0c7a12815e343a5ae68807af172a35d6c6857935
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>