i40e: count drops in netstat interface

The i40e rx_dropped counter was not showing up in netstat -i.
Add the right counter to be updated with the stats.

Change-ID: I4dd552e9995836099184f9d9a08e90edb591155f
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>