ieee1394: support for speeds greater than S800

The hard-wired configuration of the top speed (until now S800) was
unnecessary, remove it.

If the local link layer controller supports S1600 or S3200, we now
assume this speed for all present 1394b PHYs (except if they are
behind 1394a repeaters) until nodemgr figured out the actual speed
while fetching the config ROM.

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