firewire: ohci: use return value from fw_node_get()

The programming pattern, referring after increasing reference count, is
supported by fw_node_get().

This commit simplify the programming pattern.

Link: https://lore.kernel.org/r/20250908012108.514698-4-o-takashi@sakamocchi.jp


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