i40e/i40evf: fix accidental write to ITR registers

Fix a bug introduced in the force writeback code, where the interrupt
rate was set to 0 (maximum) by accident.

The driver must correctly set the NOITR fields to avoid ITR update
as a side effect of triggering the software interrupt.

Change-ID: I290851ae04ef3811c43aab5ee33242029f26c1a3
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>