Steel Tempest cooldown is reduced by 1% for every 1.72% bonus attack speed. Another easier way to think about it is for every 1% bonus attack speed he gets .58% cdr
The cooldown of Steel Tempest is unaffected by cooldown reduction and is instead reduced by 1% for every 1.72% of his bonus attack speed. This is capped at 66% cooldown reduction with 114% bonus attack speed.
At max rank, the minimum cooldown is 1.33 seconds (1.4 attack speed).
Yasuo's 4% attack speed per level is factored into Steel Tempest's cooldown reduction and cast time reduction. At level 18, Yasuo has a total of 68% bonus attack speed from leveling alone which reduces Steel Tempest's cooldown by 40%. Steel Tempest's cooldown is effectively reduced by 2.3% per level.
So to answer your question 40% attack speed would get him 40/1.72(or 40*.58) = 23.25% bonus cdr for the ability