🩹 Fix STM32 HW Serial (MarlinFirmware#26531)

Followup to MarlinFirmware#26328

Author: rmbq

Marlin/src/HAL/STM32/HardwareSerial.cpp

@@ -126,7 +126,7 @@
   void serialEvent2() __attribute__((weak));
 #endif
 #ifdef USING_HW_SERIAL3
-  HAL_HardwareSerial Serial3(USART3);
+  HAL_HardwareSerial HSerial3(USART3);
   void serialEvent3() __attribute__((weak));
 #endif
 #ifdef USING_HW_SERIAL4
@@ -147,9 +147,9 @@
 #endif
 #ifdef USING_HW_SERIAL6
   #ifdef USART6
-    HAL_HardwareSerial HSerial5(USART6);
+    HAL_HardwareSerial HSerial6(USART6);
   #else
-    HAL_HardwareSerial HSerial5(UART6);
+    HAL_HardwareSerial HSerial6(UART6);
   #endif
   void serialEvent5() __attribute__((weak));
 #endif