[ALSA] ac97_codec - fix duplicate control creation in AC97

This patch conditions AC97 control creation by whether or not the
codec is an AD18xx codec. This fixes the case where the default control
would get created and then snd_ac97_mixer_build fails out when creation
of ad18xx specific control would get attempted. This problem was found
and debuged by Marcelo Tosatti.

Signed-off-by: Jaya Kumar <jayakumar.alsa@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>