This commit adds a support for capturing PCM samples.

When opposite PCM substream is already running, available sampling rate is
limited at current one.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>