firewire: reorder struct fw_card for better cache efficiency

topology_map is by far the largest member in struct fw_card.  Move it to
the very end of the struct so that card pointer dereferences have better
chances to hit the CPU cache.

This requires to increase the topology_map backing store to the size
specified in IEEE 1394, i.e. 256 rather than 255 quadlets.  Otherwise
the topology_map response handler may access invalid memory.

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