i40e: simplify aq head-tail-len setups

Use more virtual registers to simplify code flows.

Change-Id: I32cff3818c5ca3a3792487ba4fed8f1d0ea6145a
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>