i40e/i40evf: Update check for AQ aliveness

Update the i40e_check_asq_alive check to ensure that the len register
offset is non-zero, indicating that SW has initialized the AQ.

Change-ID: I9c2e804788b4775bef9c7e80954ab004e6bdb306
Signed-off-by: Kevin Scott <kevin.c.scott@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>