firewire: core: code refactoring for the case of generation mismatch

firewire: core: code refactoring for the case of generation mismatch

Current implementation stores the bus generation at which the bus manager
contending procedure finishes. The condition for the procedure is the
mismatch of the stored generation against current bus generation.

This commit refactors the code for the contending procedure. Two existing
branches are put into a new branch to detect the generation mismatch, thus
the most of change is indentation.

Link: https://lore.kernel.org/r/20250918235448.129705-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>