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

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

This commit merges the packet handler into function for IR context
callback. The logic to parse IR context 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>