firewire: core: fix overlooked update of subsystem ABI version

firewire: core: fix overlooked update of subsystem ABI version

In kernel v6.5, several functions were added to the cdev layer. This
required updating the default version of subsystem ABI up to 6, but
this requirement was overlooked.

This commit updates the version accordingly.

Fixes: 6add87e9764d ("firewire: cdev: add new version of ABI to notify time stamp at request/response subaction of transaction#")
Link: https://lore.kernel.org/r/20250920025148.163402-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>