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.