riscv: hw: Change create_fdt() to return void

riscv: hw: Change create_fdt() to return void

There is no need to return fdt at the end of create_fdt() because
it's already saved in s->fdt.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Chih-Min Chao <chihmin.chao@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>