ALSA: fireface: add protocol-specific operation to allocate isochronous resources

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

In ALSA fireface driver, the allocation of isochronous resources is
programmed in each implementation of protocol. This commit adds
protocol-specific operation for the allocation separated from
the operation to begin session.

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