target/riscv/csr.c: use 'vlenb' instead of 'vlen'

target/riscv/csr.c: use 'vlenb' instead of 'vlen'

As a bonus, we're being more idiomatic using cpu->cfg.vlenb when
reading CSR_VLENB.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240122161107.26737-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>