osc.lua: fix frame stepping with skip_backward/forward buttons

The event responder must have up, down, or press suffix, but
940ab990558a017e45cca3daa91f65ecff170c38 removed it for frame stepping
function, making it unusable. Fix this by using the down suffix.

Fixes: 940ab990558a017e45cca3daa91f65ecff170c38