target/riscv: Fix rmw_sip, rmw_vsip, rmw_hsip vs write-only operation

target/riscv: Fix rmw_sip, rmw_vsip, rmw_hsip vs write-only operation

We distinguish write-only by passing ret_value as NULL.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210823195529.560295-17-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>