ALSA: ac97: More cleanup with snd_ctl_find_id_mixer()

There was one overlooked place to be replaced with
snd_ctl_find_id_mixer() for code simplification.

No functional change, only code refactoring.

Link: https://lore.kernel.org/r/20240304082158.8583-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>