Fix kernel-doc warning in <sound/pcm.h> and add function name to make
the kernel-doc notation complete.

Warning(include/sound/pcm.h:1081): No description found for parameter 'substream'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>