target/riscv: Fix VS mode hypervisor CSR access

target/riscv: Fix VS mode hypervisor CSR access

VS mode access to hypervisor CSRs should generate virtual, not illegal,
instruction exceptions.

Don't return early and indicate an illegal instruction exception when
accessing a hypervisor CSR from VS mode. Instead, fall through to the
`hmode` predicate to return the correct virtual instruction exception.

Signed-off-by: Dylan Reid <dgreid@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220506165456.297058-1-dgreid@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>