firewire: normalize a variable name

Standardize on  if (err)
                        handle_error;
           and  if (ret < 0)
                        handle_error;

Don't call a variable err if we store values in it which mean success.
Also, offset some return statements by a blank line since this how we do
it in drivers/firewire.

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