ALSA: opti-miro: fix OOPS if hardware is not detected

If a hardware is not detected there is a kernel crash
due to not initialized snd_miro->aci pointer. This pointer
is initialized after detection of the opti (miro) chip.

This bug was introduced by patches to expose
ACI mikser outside the snd-miro driver.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>