ALSA: dice: code refactoring to keep isochronous resources

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

This commit adds a helper function to allocate isochronous resources,
separated from operations to start packet streaming, I note that some
dice-based devices have two pair of endpoints for isochronous packet
straeming.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>