🐛 Fix FYSETC S6, S6 V2 Serial RX pins (#23642)

Marlin/src/pins/stm32f4/pins_FYSETC_S6.h

     #define Y_SERIAL_TX_PIN                 PE14
   #endif
   #ifndef Y_SERIAL_RX_PIN
-    #define Y_SERIAL_RX_PIN                 PC4
+    #define Y_SERIAL_RX_PIN                 PE13
   #endif
   #ifndef Z_SERIAL_TX_PIN
     #define Z_SERIAL_TX_PIN                 PD11

Marlin/src/pins/stm32f4/pins_FYSETC_S6_V2_0.h

 
 #if HAS_TMC_UART
   #define X_SERIAL_TX_PIN                   PE8
+  #define X_SERIAL_RX_PIN                   PE8
   #define Y_SERIAL_TX_PIN                   PC4
+  #define Y_SERIAL_RX_PIN                   PC4
   #define Z_SERIAL_TX_PIN                   PD12
+  #define Z_SERIAL_RX_PIN                   PD12
   #define E0_SERIAL_TX_PIN                  PA15
+  #define E0_SERIAL_RX_PIN                  PA15
   #define E1_SERIAL_TX_PIN                  PC5
+  #define E1_SERIAL_RX_PIN                  PC5
   #define E2_SERIAL_TX_PIN                  PE0
+  #define E2_SERIAL_RX_PIN                  PE0
 #endif
 
 //