[ALSA] fix build failure due to snd-aoa

When snd-aoa is not built or built as modules, but CONFIG_SND is yes,
kernel build fails due to a bug I introduced when adding snd-aoa. This
patch fixes it.
From: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>