firewire: core: add helper functions to access to fw_device data in fw_node structure

The data mbmer in fw_node structure is an opaque pointer, while nowadays
it is just used to refer to fw_device associated with the fw_node.

This commit redefines the opaque pointer to a pointer to fw_device
structure, and adds some helper functions to set/get it.

Link: https://lore.kernel.org/r/20250908012108.514698-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>