lal_gate: "fix" lock-ups

- it seems the chain functions must not return GST_FLOW_FLUSHING.  it seems to result in lock-ups, but I don't understand why.  I don't understand if the lockups are a bug in this element, or that we're triggering a lockup in gstreamer
- I'm inclined to believe it's the latter, that returning GST_FLOW_FLUSHING was a non-conformance bug in this element but that it had no negative side effects before while now it does