Fixes SD card use on Melzi

In the current state Marlin defines pin 24 to SDSS for Melzi boards, wich leads to a corrupt hotend temperature readout. SDSS should be set to pin 31 like the original code does. Commented out the sections that sets SDSS to 24 for melzi.
Also see http://forums.reprap.org/read.php?147,141416,141416