i40e: check for AQ timeout in aq_rc decode

Decoding the AQ return code is great except when the AQ send timed out
and there's no return code set.  This changes the handy decoder
interface to help catch and properly report the condition as a useful
errno rather than returning a misleading '0'.

Change-ID: I07a1f94f921606da49ffac7837bcdc37cd8222eb
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>