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: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>