target/riscv: Return the exception from invalid CSR accesses

target/riscv: Return the exception from invalid CSR accesses

When performing a CSR access let's return a negative exception value on
an error instead of -1. This will allow us to specify the exception in
future patches.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: a487dad60c9b8fe7a2b992c5e0dcc2504a9000a7.1597259519.git.alistair.francis@wdc.com
Message-Id: <a487dad60c9b8fe7a2b992c5e0dcc2504a9000a7.1597259519.git.alistair.francis@wdc.com>