i40e: adjust ITR max and min values

Set the ITR max and min values to match the hardware max value
and the recommended min value.  These values are shifted right
one bit because the register counts in 2 usec units, so leave
a comment to explain.

Change-ID: I289c27955cf6c566a6d21b95c3110b88cbb15dad
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>