Hey everyone, I have the same board revision (v1.2a) as @N8net and I can confirm that @jit-010101's BIOS indeed does not work, the hidden option is present but selecting it causes the BIOS to hang.
I found a working modified BIOS over in the ServeTheHome Forums though, specifically xoxer's version from this thread: https://forums.servethehome.com/index.php?threads/topton-nas-motherboard.37979/post-417984
A working download can be found a bit down the thread, specifically here. Should the link be dead aswell, I've got a copy which I could provide, just let me know. The auto-install seems to be broken if jit's BIOS is installed, so you might need to do it manually, instructions can be found in the thread aswell.
With that BIOS the hidden options are present and work, just set the DMI Link ASPM Control to [L0sL1] and the ASPM setting for each PCIe entry to [Auto], with that c8 should be possible. Some users there reportet that they needed to disable the turbo in the BIOS, that was not needed in my case though.
Additionally, the BIOS seems to crash for me after 2 - 3 minutes and the system just reboots without warning. So just be quick with changing the settings, besides that everything works fine so far (have not yet tested any long-term stability).