firewire: ohci: use devres for list of isochronous contexts

The 1394 OHCI driver allocates the list of isochronous contexts as much
as the hardware supports.

This commit utilizes managed device resource to maintain the lifetime of
list.

Link: https://lore.kernel.org/r/20230604054451.161076-7-o-takashi@sakamocchi.jp


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