firewire: core: improve reread_config_rom() interface

The return value of reread_config_rom() was a mixture of two pieces of
information: whether the function succeeded, and whether the config rom
had changed.

To clarify the semantics, and to allow returning the actual error code,
split the second information into a new output parameter.

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