i40e: Admin queue shutdown fixes

Always call the AQ call to shutdown the queue in the shutdown path.

Check ASQ is alive before issuing the AQ command since we might be
resetting to recover from a bad state in which case we should not
issue the AQ command.

Use the register variable for length so it can be used by PF, VF
and GL AQ commands.

Change-Id: Ic3d305687ea3f1a6afa84e864b7a27bd38a9af32
Signed-off-by: Anjali Singhai Jain <anjali.singhai@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>