firewire: core: code refactoring to investigate root node for bus manager

In the middle of bm_work function, both the value of gap_count and the
state of root node are investigated. Current implementation is not a good
shape since the investigation is aligned to be flat.

This commit refactors the investigation with two large branches.

Link: https://lore.kernel.org/r/20250908012108.514698-11-o-takashi@sakamocchi.jp


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