Streaming functionality can start streams when rate is given but currently
some codes are in PCM functionality.

This commit changes the way to start stream and add some arrangement
to make it easy to understand the way.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>