[ALSA] snd-aoa: not experimental

The dependencies in the soundbus Kconfig were wrong, it isn't
experimental any more.
This patch fixes that and makes it select SND_PCM too instead of
depending on it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>