firewire: core: move workqueue handler from 1394 OHCI driver to core function

firewire: core: move workqueue handler from 1394 OHCI driver to core function

In current implementation, the work item for isochronous context executes
the same procedure of fw_iso_context_flush_completions() internally. There
is a space to refactor the implementation.

This commit calls fw_iso_context_flush_completions() in the work item. It
obsoletes fw_iso_context_init_work(). It also obsoletes a pair of
disable_work_sync() and enable_work() since the usage of
test_and_set_bit_lock() mediates concurrent call already.

Link: https://lore.kernel.org/r/20240909140018.65289-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>