drivers/perf: hisi: Simplify hisi_read_sccl_and_ccl_id and its comment

hisi_read_sccl_and_ccl_id is not readable and its comment is a little
confused, so simplify the function and its comment as Mark's suggestion.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Will Deacon <will@kernel.org>