target/riscv: gdbstub: Fix dynamic CSR XML generation

target/riscv: gdbstub: Fix dynamic CSR XML generation

Since commit 605def6eeee5 ("target/riscv: Use the RISCVException enum for CSR operations")
the CSR predicate() function was changed to return RISCV_EXCP_NONE
instead of 0 for a valid CSR, but it forgot to update the dynamic
CSR XML generation codes in gdbstub.

Fixes: 605def6eeee5 ("target/riscv: Use the RISCVException enum for CSR operations")
Reported-by: Xuzhou Cheng <xuzhou.cheng@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Xuzhou Cheng <xuzhou.cheng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210615085133.389887-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>