firewire: cdev: fix back-transition for iso_resource_auto client resource

The todo member of iso_resource_auto structure represents the state of the
client resource and normally transitions in the following order:

    ISO_RES_AUTO_ALLOC -> ISO_RES_AUTO_REALLOC -> ISO_RES_AUTO_DEALLOC

However, concurrent access from the work item and the file descriptor
release function can cause the state to transition backwards from
ISO_RES_AUTO_DEALLOC to ISO_RES_AUTO_REALLOC.

Prevent the back-transition by checking the current state before
updating it in the work item.

Fixes: fcabbf40fae5 ("firewire: core: move allocation/reallocation paths into specific branch after isoc resource management in cdev")
Link: https://lore.kernel.org/r/20260922132639.191593-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>