i40e: clear AQ head and tail registers

During admin queue shutdown clear some more registers
explicitly.

Change-Id: Ifb235c691e1c55e76bf66e0642207f464153d05a
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>