firewire: support S100B...S400B and link slower than PHY

Use a speed probe to determine the speed over 1394b buses and of nodes
which report a link speed less than their PHY speed.

Log the effective maximum speed of newly created nodes in dmesg.

Also, read the config ROM (except bus info block) at the maximum speed
rather than S100.  This isn't a real optimization though because we
still only use quadlet read requests for the entire ROM.

The patch also adds support for S1600 and S3200, although such hardware
does not exist yet.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Kristian Høgsberg <krh@redhat.com>