hw/nvram/eeprom_at24c: Reorganise init to avoid overwriting values

hw/nvram/eeprom_at24c: Reorganise init to avoid overwriting values

The init_rom[] can write values to the beginning of the memory but
these are overwritten by values from a backing file that covers the
whole memory.

init_rom[] is used only if there's no backing file (provides default
content) but should not overwrite backing file content (especially
leaving the file unchanged and only change it in memory).
Do the init_rom[] handling only if it would not be overwritten.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <fd8e0478febd60d5f48c58bc77c60e043d1c3cdc.1740839457.git.balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>