Update pins_PRINTRBOARD.h

Marlin/pins_PRINTRBOARD.h

@@ -36,6 +36,8 @@
 #endif
 
 #define BOARD_NAME         "Printrboard"
+
+#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 // Disable JTAG pins so they can be used for the Extrudrboard