This commit is a part of preparation to perform allocation/release
of isochronous resources in pcm.hw_params/hw_free callbacks.

At present, several operations are done in pcm.prepare callback. To
reduce load of the callback, This commit splits out an operation to
set packet format in pcm.hw_params callback.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>