ALSA: fix kernel-doc build warning

Fix kernel-doc build error. A symbol that ends with an underscore
character ('_') has special meaning in reST (reStructuredText), so add
a '*' to prevent this error and to indicate that there are several of
these values to choose from.

../sound/core/jack.c:312: ERROR: Unknown target name: "snd_jack_btn".
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/jack.c

@@ -310,7 +310,7 @@ EXPORT_SYMBOL(snd_jack_set_parent);
  * @type:    Jack report type for this key
  * @keytype: Input layer key type to be reported
  *
- * Map a SND_JACK_BTN_ button type to an input layer key, allowing
+ * Map a SND_JACK_BTN_* button type to an input layer key, allowing
  * reporting of keys on accessories via the jack abstraction.  If no
  * mapping is provided but keys are enabled in the jack type then
  * BTN_n numeric buttons will be reported.