firewire: move fw_device() and fw_unit() to use container_of_const()

firewire: move fw_device() and fw_unit() to use container_of_const()

The driver core is changing to pass some pointers as const, so move
fw_device() and fw_unit() functions to use container_of_const() to
handle this change.

fw_device() and fw_unit() now properly keeps the const-ness of the
pointer passed into it, while as before it could be lost.

This also required turning fw_parent_device() into a macro to preserve
the const-ness of the pointer passed into it if necessary.

Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230111113018.459199-5-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>