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: NTakashi Iwai <tiwai@suse.de>

sound/pci/ac97/ac97_codec.c

@@ -1907,7 +1907,7 @@ static int ac97_reset_wait(struct snd_ac97 *ac97, int timeout, int with_modem)
  * write).  The other callbacks, wait and reset, are not mandatory.
  * 
  * The clock is set to 48000.  If another clock is needed, set
- * (*rbus)->clock manually.
+ * ``(*rbus)->clock`` manually.
  *
  * The AC97 bus instance is registered as a low-level device, so you don't
  * have to release it manually.