Fix misc. warnings (#20715)

Marlin/src/HAL/shared/backtrace/unwarm_arm.cpp

 }
 
 UnwResult UnwStartArm(UnwState * const state) {
-  bool found = false;
   uint16_t   t = UNW_MAX_INSTR_COUNT;
 
-  do {
+  for (;;) {
     uint32_t instr;
 
     /* Attempt to read the instruction */
 
     if (--t == 0) return UNWIND_EXHAUSTED;
 
-  } while (!found);
+  }
 
   return UNWIND_UNSUPPORTED;
 }

Marlin/src/HAL/shared/backtrace/unwarm_thumb.cpp

 }
 
 UnwResult UnwStartThumb(UnwState * const state) {
-  bool found = false;
   uint16_t t = UNW_MAX_INSTR_COUNT;
   uint32_t lastJumpAddr = 0;  // Last JUMP address, to try to detect infinite loops
   bool loopDetected = false;  // If a loop was detected
 
-  do {
+  for (;;) {
     uint16_t instr;
 
     /* Attempt to read the instruction */
 
     if (--t == 0) return UNWIND_EXHAUSTED;
 
-  } while (!found);
+  }
 
   return UNWIND_SUCCESS;
 }

Marlin/src/lcd/dogm/u8g_dev_ssd1306_sh1106_128x64_I2C.cpp

 
 uint8_t u8g_WriteEscSeqP_2_wire(u8g_t *u8g, u8g_dev_t *dev, const uint8_t *esc_seq) {
   uint8_t is_escape = 0;
-  uint8_t value;
   for (;;) {
-    value = u8g_pgm_read(esc_seq);
+    uint8_t value = u8g_pgm_read(esc_seq);
     if (is_escape == 0) {
       if (value != 255) {
         if (u8g_WriteByte(u8g, dev, value) == 0 )