ALSA: timer: Introduce disconnect op to snd_timer_instance

Instead of the previous ugly hack, introduce a new op, disconnect, to
snd_timer_instance object for handling the wake up of pending tasks
more cleanly.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=109431


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