ALSA: timer: Revert active callback sync check at close

This is essentially a revert of the commit a7588c89 ("ALSA: timer:
Check ack_list emptiness instead of bit flag").  The intended change
by the commit turns out to be insufficient, as snd_timer_close*()
always calls snd_timer_stop() that deletes the ack_list beforehand.

In theory, we can change the behavior of snd_timer_stop() to sync the
pending ack_list, but this will become a deadlock for the callback
like sequencer that calls again snd_timer_stop() from itself.  So,
reverting the change is a more straightforward solution.

Fixes: a7588c89 ("ALSA: timer: Check ack_list emptiness instead of bit flag")
Reported-by:  <syzbot+58813d77154713f4de15@syzkaller.appspotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>