Revert "firewire: core: use mutex to coordinate concurrent calls to flush completions"

This reverts commit d9605d67562505e27dcc0f71af418118d3db91e5, since this
commit is on the following reverted changes.

Link: https://lore.kernel.org/r/20240912133038.238786-2-o-takashi@sakamocchi.jp


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