You cannot pass arguments/parameters to a function when calling it from a button.
But what you can do is the following (writing this from the top of my head so no guarantee that it will work):
Code:
function set_difficulty(button_number, panel)
{
  if(button_number == 1)
  {
    // easy
  }
  if(button_number == 2)
  ...
}

// in your button do this (e.g.)
button ( 325, 300, easy_roll, easy_norm, easy_roll, set_difficulty, NULL, NULL);


Btw: are you using c-script or lite-c?