[ALSA] add number of periods constraint to snd-aoa

The aoa driver is not specifying constraints on number of periods, and, it
seems, it might end with a non-integer number, which it cannot deal with.
Fix by adding a proper constraint.

Signed-off-by: Heikki Lindholm <holindho@cs.helsinki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>