i40e/i40evf: add Tx pre queue disable function

Add a function which indicates our intention to enable or disable a particular
Tx queue. Also add a function to notify the device's Tx unit that we're about
to enable or disable a Tx queue.

Change-ID: I6adf3cbb5bb3e3c984d1ec969e06577c19ef296d
Signed-off-by: Matt Jared <matthew.a.jared@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>