When ASoC driver is unbound dynamically during its operation (i.e. a
kind of hot-unplug), we may hit Oops due to the resource access after
the release by a delayed work, something like:

  Unable to handle kernel paging request at virtual address dead000000000220
  ....
  PC is at soc_dapm_dai_stream_event.isra.14+0x20/0xd0
  LR is at snd_soc_dapm_stream_event+0x74/0xa8
  ....
  [<ffff000008715610>] soc_dapm_dai_stream_event.isra.14+0x20/0xd0
  [<ffff00000871989c>] snd_soc_dapm_stream_event+0x74/0xa8
  [<ffff00000871b23c>] close_delayed_work+0x3c/0x50
  [<ffff0000080bbd6c>] process_one_work+0x1ac/0x318
  [<ffff0000080bbf20>] worker_thread+0x48/0x420
  [<ffff0000080c201c>] kthread+0xfc/0x128
  [<ffff0000080842f0>] ret_from_fork+0x10/0x18

For fixing the race, this patch adds a sync-point in pcm private_free
callback to finish the delayed work before actually releasing the
resources.
Reported-by: Hiep Cao Minh <cm-hiep@jinso.co.jp>
Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>