target/riscv: Check the correct exception cause in vector GDB stub

target/riscv: Check the correct exception cause in vector GDB stub

After RISCVException enum is introduced, riscv_csrrw_debug() returns
RISCV_EXCP_NONE to indicate there's no error. RISC-V vector GDB stub
should check the result against RISCV_EXCP_NONE instead of value 0.
Otherwise, 'E14' packet would be incorrectly reported for vector CSRs
when using "info reg vector" GDB command.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-Id: <20220918083245.13028-1-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>