[ALSA] aoa: add locking to tas codec

Looks like I completely forgot to do this. This patch adds locking to
the tas codec so two userspace programs can't hit the controls at the
same time. Tested on my powerbook, but I obviously can't find any
problems even without it since it doesn't do SMP.

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