Merge pull request #2132 from thinkyhead/fix_M120_M121

M120 and M121 are reversed

Marlin/Marlin_main.cpp

 /**
  * M120: Enable endstops
  */
-inline void gcode_M120() { enable_endstops(false); }
+inline void gcode_M120() { enable_endstops(true); }
 
 /**
  * M121: Disable endstops
  */
-inline void gcode_M121() { enable_endstops(true); }
+inline void gcode_M121() { enable_endstops(false); }
 
 #ifdef BLINKM