Use elif and parentheses in conditionals

Marlin/Conditionals.h

   #ifndef POWER_SUPPLY
     #define POWER_SUPPLY 1
   #endif
-  #if (POWER_SUPPLY == 1)   // 1 = ATX
+  #if (POWER_SUPPLY == 1)     // 1 = ATX
     #define PS_ON_AWAKE  LOW
     #define PS_ON_ASLEEP HIGH
-  #endif
-  #if (POWER_SUPPLY == 2)   // 2 = X-Box 360 203W
+  #elif (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