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 <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-id: 20210823195529[email protected]
Signed-off-by: Alistair Francis <[email protected]>