cmd: fat: Use do_save() for fatwrite

do_save() function defined in fs.c also supports FAT file system
re-use the same for fatwrite command.

Also fix the FAT test script to match the expected output.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>