target/riscv: Use PRV_RESERVED instead of PRV_H

target/riscv: Use PRV_RESERVED instead of PRV_H

PRV_H has no real meaning, but just a reserved privilege mode currently.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230407014743.18779-3-liweiwei@iscas.ac.cn>
[ Changes by AF:
 - Convert one missing use of PRV_H
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>