ALSA: au88x0: fix .pointer callback

Appearently, the used mask in the .pointer callback is invalid. It should
be in period_bytes range. The period_bytes is pow(2), so simple bitwise
operation is used.

Idea was taken from ALSA bug#4455.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>