Fix typo from long ago

Firmware/MarlinSerial.h

@@ -28,10 +28,10 @@
 #endif
 
 // The presence of the UBRRH register is used to detect a UART.
-#if ((port == 0 && (defined(UBRRH) || defined(UBRR0H))) || \
-	(port == 1 && defined(UBRR1H)) || \
-	(port == 2 && defined(UBRR2H)) || \
-	(port == 3 && defined(UBRR3H)))
+#if ((SERIAL_PORT == 0 && (defined(UBRRH) || defined(UBRR0H))) || \
+	(SERIAL_PORT == 1 && defined(UBRR1H)) || \
+	(SERIAL_PORT == 2 && defined(UBRR2H)) || \
+	(SERIAL_PORT == 3 && defined(UBRR3H)))
 #define HAS_UART
 #endif