IEEE 1394 up to edition 2008
defines only up to three self IDs per node).  More than 252 self IDs
would only happen if the self ID receive DMA unit malfunctioned, which
would likely be caught by other self ID buffer checks.  However, check
it early to be sure.  More than 253 quadlets would overflow the Topology
Map CSR.

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