i40e: check desc pointer before printing

Check that the descriptors were allocated before trying to dump
them to the logfile.  While we're there, de-trick-ify the code
so as to be easier to read and not abusing the types and unions.

Change-ID: I22898f4b22cecda3582d4d9e4018da9cd540f177
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>