hwtracing: hisi_ptt: Don't try to attach a task

PTT is an uncore PMU and shouldn't be attached to any task. Block
the usage in pmu::event_init().

Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20231010084731.30450-5-yangyicong@huawei.com