Use elif and parentheses in conditionals

Marlin/Conditionals.h

@@ -268,12 +268,11 @@
   #if (POWER_SUPPLY == 1)     // 1 = ATX
     #define PS_ON_AWAKE  LOW
     #define PS_ON_ASLEEP HIGH
-  #endif
+  #elif (POWER_SUPPLY == 2)   // 2 = X-Box 360 203W
-  #if (POWER_SUPPLY == 2)   // 2 = X-Box 360 203W
     #define PS_ON_AWAKE  HIGH
     #define PS_ON_ASLEEP LOW
   #endif
-  HAS_POWER_SWITCH = POWER_SUPPLY > 0 && defined(PS_ON_PIN) && PS_ON_PIN >= 0
+  HAS_POWER_SWITCH = (POWER_SUPPLY > 0 && defined(PS_ON_PIN) && PS_ON_PIN >= 0)
 
   /**
    * Temp Sensor defines