target/riscv: Don't wrongly override isa version

target/riscv: Don't wrongly override isa version

For some cpu, the isa version has already been set in cpu init function.
Thus only override the isa version when isa version is not set, or
users set different isa version explicitly by cpu parameters.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210811144612.68674-1-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>