[ALSA] ac97 - Fix power_save option value as time-out

The power_save option was set as boot although it was meant to be a
timeout value like the same option of snd-hda-intel originally.
Now fixed to the same style.

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