fs: littlefs: Use flash map macro in automount

To identify the flash partition id, the macro DT_FIXED_PARTITION_ID()
was used previously.

Now the macro FLASH_AREA_ID() is used. This also supports the usecase
when a different flash map implementation is used that redefines the
macros in include/storage/flash_map.h, e.g. the nordic partition
manager.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>