target/riscv: Fix mstatus dirty mask

target/riscv: Fix mstatus dirty mask

This is meant to mask off the hypervisor bits, but a typo caused it to
mask MPP instead.

Fixes: 1f0419cb04 ("target/riscv: Allow setting mstatus virtulisation bits")
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>