fs/littlefs: Use flash_area_flatten in lfs_api_erase

LittleFS is based on program-erase devices and require that
characteristics to work; the commit replaces flash_area_erase
with flash_area_flatten to allow LFS work on devices that
do not provide erase callback.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>