asix-sigma: Disable support for triggers, they don't work right now

This works around bug #359. Triggers currently are not operational for
Asix Sigma. Don't claim support in the driver so that UIs won't use the
feature. Yet allow research in this issue, by concentrating the switch
for the feature's support in a central location.

Add/update a comment and unobfuscate an error code path while we are here.