i40e: simplify error messages for dump descriptor

debugfs fixes: We don't really need to give usage messages for data errors,
only for invalid command errors.

Change-Id: If3f74ac49e43c3ced7fd388323fa738ac145e055
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>