ALSA: firewire-lib: refactoring to obsolete IT packet handler

As a result of heavy refactoring based on IT packet header, the packet
handler becomes simpler.

This commit merges the packet handler into function for IT context
callback. The logic to build IT packet header and tracepoints event is
split to a function.

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