You shouldn't be required to suspend/resume the listener when perform an AsyncOS upgrade.

Quoting the Upgrade Instructions section of the 6.5 release notes:

3. Suspend all listeners.

4. Wait for the queue to emtpy.

5. From the System Administration tab, select the System Upgrade page.

I have to agree with randhall. If the instructions say to suspend the listeners and then make a specific reference to using the GUI to start the upgrade, then there should be a GUI method for suspending the listeners.