i40evf: enable adaptive interrupt throttling

All of the code to support adaptive interrupt throttling is already in
the interrupt handler, it just needs to be enabled. Fill out the data
structures properly to make it happen. Single-flow traffic tests may
show slightly lower throughput, but interrupts per second will drop by
about 75%.

Change-ID: I9cd7d42c025b906bf1bb85c6aeb6112684aa6471
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>