i40evf: Force Tx writeback on ITR

This patch forces Tx descriptor writebacks on ITR by kicking
off the SWINT interrupt when we notice that there are non-cache-aligned
Tx descriptors waiting in the ring while interrupts are disabled
under NAPI.

Change-ID: dd6d9675629bf266c7515ad7a201394618c35444
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>