Fix STM32F1 SD-based EEPROM emulation (#13475)

If `openFile` is given `true`, then it has read-only access. Because all the writing occurs on the next line, this breaks SD card as EEPROM.

Marlin/src/HAL/HAL_STM32F1/persistent_store_sdcard.cpp

 
 bool PersistentStore::access_finish() {
   if (!card.isDetected()) return false;
-  card.openFile(eeprom_filename, true);
+  card.openFile(eeprom_filename, false);
   int16_t bytes_written = card.write(HAL_STM32F1_eeprom_content, HAL_STM32F1_EEPROM_SIZE);
   card.closefile();
   return (bytes_written == HAL_STM32F1_EEPROM_SIZE);