firewire: 'add CSR_... support' addendum

Add a comment on which of the conflicting NODE_IDS specifications we
implement.  Reduce a comment on rather irrelevant register bits that can
all be looked up in the spec (or from now on in the code history).
Directly include the required indirectly included bug.h.

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