[2.0.x] Change MKS SBASE default serial port to avoid confusion (#11301)

Marlin/src/config/examples/Mks/Sbase/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT 0
+#define SERIAL_PORT -1
 
 /**
  * Select a secondary serial port on the board to use for communication with the host.
  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-//#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 0
 
 /**
  * This setting determines the communication speed of the printer.