From what I can see you need to add another function to reset the values e.g.

global proc reset()
{
setAttr $xOffset.horizontalFilmOffset 0;
setAttr $yOffset.verticalFilmOffset 0;
}

The only problem you will have is that $xOffset and $yOffset arent visible to this function as they are declared "in scope". So I would declare them as global or outside of that scope (or you will have to pass them in as params). That way it will work. I would also save off the users initial settings into variables and give them the option of restoring them rather than going back to 0.

One thing I should point out is that your variable names for the camera re really bad. I wouldn't bother with $xOffset and $yOffset (or at least change their names cause they are confusing )

Hope this helps a bit.

Alan