i40e/i40evf: set proper default for ITR registers

Ethtool consistently reports 0 values for our ITR settings because
we never actually set them. Fix this by setting the default values
to the specified default values.

Change-ID: I2832406a66f7140f2b1230945d6ff6cbf77467c8
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>