ALSA: timer: Make sure to clear pending ack list

When a card is under disconnection, we bail out immediately at each
timer interrupt or tasklet.  This might leave some items left in ack
list.  For a better integration of the upcoming change to check
ack_list emptiness, clear out the whole list upon the emergency exit
route.

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