i40e: rename variable to prevent clash of understanding

This code returns something that becomes the errno value from ethtool and
passes around a pointer to an errno variable.  This patch changes the name
slightly to differentiate it from the actual user errno variable.

Change-ID: Idaa37845c069e66f4cea072e90f471bb2142454d
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>