i40e: rename debugfs clear_stats option

Change debugfs command from "clear_stats pf" to "clear_stats port"
to be clearer what the action is.  Also, limit the action to the
base PF, not the NPAR partitions.

Change-ID: I22aa39c0962d83a83a985097b1000ed7f8c66f3f
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>