The line onclick = writeLED(1,1)
means that you want to immediately execute the function writeLED(arg1, arg2) with arguments 1, 1 and assign the return value; you need to instead create a function that will execute with those arguments and assign that. The topmost answer gave one example - another is to use the bind() function like so:
var writeLEDWithSpecifiedArguments = writeLED.bind(this, 1,1);
document.getElementById('buttonLED'+id).onclick = writeLEDWithSpecifiedArguments;