firewire: core: code refactoring to split contention procedure for bus manager

The precedure to contend for bus manager has much code. It is better to
split it into a helper function.

This commit refactors in the point.

Link: https://lore.kernel.org/r/20250918235448.129705-5-o-takashi@sakamocchi.jp


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