sound: add missing pcm kernel-doc

Fix alsa kernel-doc warning in linux-next:

Warning(linux-next-20081016//sound/core/pcm_misc.c:327): No description found for parameter 'samples'
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/pcm_misc.c

@@ -319,6 +319,7 @@ EXPORT_SYMBOL(snd_pcm_format_physical_width);
 /**
  * snd_pcm_format_size - return the byte size of samples on the given format
  * @format: the format to check
+ * @samples: sampling rate
  *
  * Returns the byte size of the given samples for the format, or a
  * negative error code if unknown format.