target/riscv: Assert that the CSR numbers will be correct

target/riscv: Assert that the CSR numbers will be correct

The CSRs will always be between either CSR_MHPMCOUNTER3 and
CSR_MHPMCOUNTER31 or CSR_MHPMCOUNTER3H and CSR_MHPMCOUNTER31H.

So although ctr_index can't be negative, Coverity doesn't know this and
it isn't obvious to human readers either. Let's add an assert to ensure
that Coverity knows the values will be within range.

To simplify the code let's also change the RV32 adjustment.

Fixes: Coverity CID 1523910
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240108001328.280222-2-alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>