ALSA: snd_ctl_activate_id(): Fix index look-up

We want to know the offset for the id that was passed to the function, not
the offset of the first id of the control (which is always 0).

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>