hw/riscv: use MachineState::fdt in riscv_socket_fdt_write_id()

hw/riscv: use MachineState::fdt in riscv_socket_fdt_write_id()

There's no need to use a MachineState pointer and a fdt pointer now that
all RISC-V machines are using the FDT from the MachineState.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230111170948.316276-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>