ALSA: firewire-lib: code refactoring for parser of IR context header

This commit refactors regarding to function argument for the length of
isochronous packet payload.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518130048.146596-6-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>