Fix Ender-3 V2 DWIN LPC signed warnings (#20786)

Marlin/src/lcd/dwin/e3v2/dwin.h

     float Move_E_scale    = 0;
   #endif
   float offset_value      = 0;
-  TERN_(__STM32F1__, signed)
-  char show_mode          = 0; // -1: Temperature control    0: Printing temperature
+  int8_t show_mode        = 0; // -1: Temperature control    0: Printing temperature
 } HMI_value_t;
 
 #define DWIN_CHINESE 123