ALSA: firewire-lib: add AMDTP domain structure to handle several isoc contexts

This commit adds 'struct amdtp_domain' structure. This structure
has list of instance of AMDTP stream to handle a couple of
isochronous contexts.

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