firewire: core: remove useless lockdep_assert_held()

The bm_work work item should be scheduled after holding fw_card reference
counting. At a commit 25feb1a9 ("firewire: core: use cleanup function
in bm_work"), I misinterpreted it as fw_card spinlock and inserted
lockdep_assert_hold() wrongly.

This commit removes the useless line.

Fixes: 25feb1a9 ("firewire: core: use cleanup function in bm_work")
Link: https://lore.kernel.org/r/20250911221312.678076-1-o-takashi@sakamocchi.jp


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