Fix problems with LPC1768 EEPROM flash emulation (#12503)

* Remove duplicate calls to PrepareSector

Some flash memory API calls require that a call to `PrepareSector` is done before use. However this call is already made by the LPC1768 framework so the calls in this code are not required.

* Ensure correct alignment of RAM buffer

The LPC176X flash API requires that the RAM buffer used for write operations must be word-aligned. This change ensures that this is the case.

Marlin/src/HAL/HAL_LPC1768/persistent_store_flash.cpp

 #define EEPROM_ERASE (0xff)
 #define SLOT_ADDRESS(sector, slot) (((uint8_t *)SECTOR_START(sector)) + slot * EEPROM_SIZE)
 
-static uint8_t ram_eeprom[EEPROM_SIZE];
+static uint8_t ram_eeprom[EEPROM_SIZE] __attribute__((aligned(4))) = {0};
 static bool eeprom_dirty = false;
 static int current_slot = 0;
 
     if (--current_slot < 0) {
       // all slots have been used, erase everything and start again
       __disable_irq();
-      PrepareSector(EEPROM_SECTOR, EEPROM_SECTOR);
       status = EraseSector(EEPROM_SECTOR, EEPROM_SECTOR);
       __enable_irq();
 
     }
 
     __disable_irq();
-    PrepareSector(EEPROM_SECTOR, EEPROM_SECTOR);
     status = CopyRAM2Flash(SLOT_ADDRESS(EEPROM_SECTOR, current_slot), ram_eeprom, IAP_WRITE_4096);
     __enable_irq();