ALSA: firewire-lib: split helper function to check incoming CIP header

A parser for CIP header in incoming packet is enough large.

This commit splits it into a helper function to better looks of packet
handler.

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