firewire: core: code refactoring whether root node is cycle master capable

The check of cycle master capability in root node is currently just in a
condition branch. In this case, the required variable should be within the
branch.

This commit is just for the purpose.

Link: https://lore.kernel.org/r/20250908012108.514698-12-o-takashi@sakamocchi.jp


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