target/riscv: Fix definition of MSTATUS_TW and MSTATUS_TSR

The TW and TSR fields should be bits 21 and 22 and not 30/29.
This was found while comparing QEMU behaviour against the sail formal
model (https://github.com/rems-project/sail-riscv/

).

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20201130170117.71281-1-Alexander.Richardson@cl.cam.ac.uk
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>