ALSA: firewire-lib: obsolete macro for header of IT context

The header size parameter of 1394 OHCI IT context has no effect.
Besides the header size of queued packet is different from context
header.

This commit cleans codes relevant to these two headers.

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