firewire: core: add function variants for isochronous context creation

firewire: core: add function variants for isochronous context creation

The fw_iso_callback union was added by a commit ebe4560ed5c ("firewire:
Remove function callback casts") to remove function pointer cast.

That change affected the cdev layer of the core code, but it is more
convenient for fw_iso_context_create() to accept the union directly.

This commit renames and changes the existing function to take the union
argument, and add static inline wrapper functions as variants.

Link: https://lore.kernel.org/r/20260117142823.440811-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>