In the previous commit, the workqueue is added per the instance of fw_card
structure for isochronous contexts. The workqueue is designed to be used by
the implementation of fw_card_driver structure underlying the fw_card.

This commit adds some local APIs to be used by the implementation.
Tested-by: Edmund Raile <edmund.raile@protonmail.com>
Link: https://lore.kernel.org/r/20240904125155.461886-3-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>