i40e/i40evf: control auto ITR through ethtool

For all of our supported kernels, ethtool allows us to directly control
adaptive ITR instead of just faking it with an ITR value. Support this
capability so that user knows explicitly when ITR is being controlled
dynamically. Suggested by Ben Hutchings.

CC: Ben Hutchings <ben@decadent.org.uk>
Change-ID: Iae6b79c5db767a63d22ecd9a9c24acaff02a096e
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>