i40e: remove hardcode of stats struct size in ethtool

Base the queue stats length on the queue stats struct rather than
assuming it is 2 fields.  This is in prep for adding data fields
which are configuration dependent.

Change-ID: I937f471f389d2e0f8cec733960c5d9a06b14f3ec
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>