ALSA: firewire-tascam: code refactoring for reservation of isochronous resources

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

This commit applies minor code refactoring for a helper function to
allocate isochronous resources.

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