i40e/base: check for AQ timeout

i40e/base: check for AQ timeout

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 an
useful error number rather than returning a misleading '0'.

Test report: http://www.dpdk.org/ml/archives/dev/2015-May/017384.html

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>
Tested-by: Min Cao <min.cao@intel.com>