firewire: core: clarify generate_config_rom usage

Move the static config ROM buffer into the scope of the two callers of
generate_config_rom().  That way the ROM length can be passed over as
return value rather than through a pointer argument.

It also becomes more obvious that accesses to the config ROM buffer have
to be serialized and how this is accomplished.  And firewire-core.ko
shrinks a bit as well.

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