target/riscv: Fix hgeie, hgeip

target/riscv: Fix hgeie, hgeip

We failed to write into *val for these read functions;
replace them with read_zero.  Only warn about unsupported
non-zero value when writing a non-zero value.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210823195529.560295-18-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>