iio: trigger: sysfs: fix use-after-free on remove

Ensure that the irq_work has completed before the trigger is freed.

 ==================================================================
 BUG: KASAN: use-after-free in irq_work_run_list
 Read of size 8 at addr 0000000064702248 by task python3/25

 Call Trace:
  irq_work_run_list
  irq_work_tick
  update_process_times
  tick_sched_handle
  tick_sched_timer
  __hrtimer_run_queues
  hrtimer_interrupt

 Allocated by task 25:
  kmem_cache_alloc_trace
  iio_sysfs_trig_add
  dev_attr_store
  sysfs_kf_write
  kernfs_fop_write_iter
  new_sync_write
  vfs_write
  ksys_write
  sys_write

 Freed by task 25:
  kfree
  iio_sysfs_trig_remove
  dev_attr_store
  sysfs_kf_write
  kernfs_fop_write_iter
  new_sync_write
  vfs_write
  ksys_write
  sys_write

 ==================================================================

Fixes: f38bc926

 ("staging:iio:sysfs-trigger: Use irq_work to properly active trigger")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20220519091925.1053897-1-vincent.whitchurch@axis.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>