Apply TERN to compact code (#17619)

Marlin/src/feature/leds/leds.h

@@ -34,7 +34,9 @@
 #endif
 
 // A white component can be passed
-#define HAS_WHITE_LED EITHER(RGBW_LED, NEOPIXEL_LED)
+#if EITHER(RGBW_LED, NEOPIXEL_LED)
+  #define HAS_WHITE_LED 1
+#endif
 
 /**
  * LEDcolor type for use with leds.set_color
@@ -85,9 +87,7 @@ typedef struct LEDColor {
 
   LEDColor& operator=(const uint8_t (&rgbw)[4]) {
     r = rgbw[0]; g = rgbw[1]; b = rgbw[2];
-    #if HAS_WHITE_LED
-      w = rgbw[3];
-    #endif
+    TERN_(HAS_WHITE_LED, w = rgbw[3]);
     return *this;
   }