Change parameter name to idx instead of index for compatibility with nesC 1.3. In...

Change parameter name to idx instead of index for compatibility with nesC 1.3.  In nesC 1.3, parameter names are no longer renamed in interfaces, and 'index' is an illegal parameter name because it shadows the *built-in* definition of index from gcc.  This has been fixed in newer versions of gcc, but the msp430-gcc we use (and have to use) is pretty old.  Moral of the story.... don't use index as a paramter name.