firewire: ohci: reduce the size of common context structure by extracting members into AT structure

In commit 386a4153 ("firewire: ohci: cache the context run bit"), a
running member was added to the context structure to cache the running
state of a given DMA context. Although this member is accessible from IR,
IT, and AT contexts, it is currently used only by the AT context.

Additionally, the context structure includes a work item, which is also
used by the AT context. Both members are unnecessary for IR and IT
contexts.

This commit refactors the code by moving these two members into a new
structure specific to AT context.

Link: https://lore.kernel.org/r/20250710131916.31289-1-o-takashi@sakamocchi.jp


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