firewire: core: add fw_iso_context_create() variant with header storage size
This commit adds a new variant of fw_iso_context_create() that allows
specifying the size of the isochronous context header storage at
allocation time.
Link: https://lore.kernel.org/r/20260117142823.440811-9-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>