firewire: core: make address handler length 64 bits

The type of the length field of the fw_address_handler structure was
size_t, which restricted it to 32 bits on 32-bit architectures.

While making it u32 would match the userspace API, all calculations on
this field use 64 bits anyway, and the ability to use 4 GB or larger
address ranges is useful in the kernel.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>