firewire: core: remove useless generation check

firewire: core: remove useless generation check

Two functions, fw_core_handle_bus_reset() and bm_work(), are serialized
by a commit 3d91fd440cc7 ("firewire: core: disable bus management work
temporarily during updating topology"). Therefore the generation member
of fw_card is immutable in bm_work().

This commit removes useless generation check in bm_work().

Link: https://lore.kernel.org/r/20250918235448.129705-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>