ALSA: timer: Check ack_list emptiness instead of bit flag

For checking the pending timer instance that is still left on the
timer object that is being closed, we set/clear a bit flag
SNDRV_TIMER_IFLG_CALLBACK around the call of callbacks.  This can be
simplified by replace with the list_empty() call for ti->ack_list.
This covers the existence more comprehensively and safely.

A gratis bonus is that we can get rid of SNDRV_TIMER_IFLG_CALLBACK bit
flag definition as well.

Signed-off-by: Takashi Iwai <tiwai@suse.de>