ALSA: fix core/vmaster.c kernel-doc warning

Fix kernel-doc warning in sound/core/vmaster.c:

Warning(sound/core/vmaster.c:429): No description found for parameter 'private_data'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/vmaster.c

@@ -419,6 +419,7 @@ EXPORT_SYMBOL(snd_ctl_make_virtual_master);
  * snd_ctl_add_vmaster_hook - Add a hook to a vmaster control
  * @kcontrol: vmaster kctl element
  * @hook: the hook function
+ * @private_data: the private_data pointer to be saved
  *
  * Adds the given hook to the vmaster control element so that it's called
  * at each time when the value is changed.