firewire: core: use macro expression for not-registered state of BUS_MANAGER_ID

The value of BUS_MANAGER_ID register has 0x3f when no node_id is
registered. Current implementation uses hard-coded numeric literal but
in the case the macro expression is preferable since it is easy to
distinguish the state from node ID mask.

This commit applies the idea.

Link: https://lore.kernel.org/r/20250913105737.778038-3-o-takashi@sakamocchi.jp


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