firewire: core: utilize cleanup function to release workqueue in error path

The helper macro, retain_and_null_ptr(), introduced by a commit
092d00ea ("cleanup: Provide retain_and_null_ptr()") in v6.16 kernel,
is useful in the error path to release the part of structure member.

This commit uses the relatively new function.

Link: https://lore.kernel.org/r/20250908012108.514698-3-o-takashi@sakamocchi.jp


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