The constraint for `rdinstreth` was comparing the csr number to 0xc80,
which is `cycleh` instead. Fix this.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>