ALSA: firewire-lib: code refactoring to start several IT/IR contexts

ALSA: firewire-lib: code refactoring to start several IT/IR contexts

It's several hundred cycles from starting isochronous contexts and the
actual cycle to start processing content of packet. It's useless to
start the context for IRQ target apart from the other contexts.

This commit refactors helper function to start AMDTP domain in the point.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210520040154.80450-7-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>