ALSA: ac97: Fix kernel-doc error with sphinx formatter

Sphinx takes a word like (*foo)->bar in the kernel-doc comments as a
part of the emphasized marker, and complains like

  ./sound/pci/ac97/ac97_codec.c:1908: WARNING: Inline emphasis start-string without end-string.

For avoiding this, wrap it with the quotes (``) in the comment.

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