i40e: debugfs fixups

debugfs fixes for issues found by coverity.

This issue was identified by the coverity checker, reported by Hannes Frederic
Sowa.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: Hannes Frederic Sowa <hannes@stressinduktion.org>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>