Apply TERN to compact code (#17619)

Marlin/src/HAL/LPC1768/HAL.cpp

@@ -70,9 +70,7 @@ int16_t PARSED_PIN_INDEX(const char code, const int16_t dval) {
 void flashFirmware(const int16_t) { NVIC_SystemReset(); }
 
 void HAL_clear_reset_source(void) {
-  #if ENABLED(USE_WATCHDOG)
-    watchdog_clear_timeout_flag();
-  #endif
+  TERN_(USE_WATCHDOG, watchdog_clear_timeout_flag());
 }
 
 uint8_t HAL_get_reset_source(void) {