🐛 Fix JyersUI for LPC176x (#22745)

Co-authored-by: Scott Lahteine <github@thinkyhead.com>

Marlin/src/HAL/LPC1768/HAL.h

@@ -107,7 +107,7 @@ extern DefaultSerial1 USBSerial;
     #error "LCD_SERIAL_PORT must be from 0 to 3. You can also use -1 if the board supports Native USB."
   #endif
   #if HAS_DGUS_LCD
-    #define SERIAL_GET_TX_BUFFER_FREE() MSerial0.available()
+    #define SERIAL_GET_TX_BUFFER_FREE() LCD_SERIAL.available()
   #endif
 #endif
 

Marlin/src/HAL/LPC1768/MarlinSerial.h

@@ -46,6 +46,8 @@ public:
 
   void end() {}
 
+  uint8_t availableForWrite(void) { /* flushTX(); */ return TX_BUFFER_SIZE; }
+
   #if ENABLED(EMERGENCY_PARSER)
     bool recv_callback(const char c) override;
   #endif