Modify the logic in i40e_determine_queue_usage() so that
SR-IOV doesn't get turned off unnecessarily.

Change-ID: I86ca304fa9f742a50e9ea831b887f358a6a9d53d
Signed-off-by: Frank Zhang <frank_1.zhang@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>