target/riscv/csr.c: use riscv_cpu_cfg() to avoid env_cpu() pointers

target/riscv/csr.c: use riscv_cpu_cfg() to avoid env_cpu() pointers

A common trend in this file is to retrieve a RISCVCPU pointer by first
retrieving a CPUState pointer via env_cpu(). The CPU pointer is used
only to access the RISCVCPUConfig object and nothing else.

Let's use riscv_cpu_cfg() to access what we need directly without these
2 pointers.

Suggested-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20230224174520.92490-4-dbarboza@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>