firewire: tone down some diagnostic log messages

The "skipped bus generations" message was added together with the
respective fw_device retaining/ reviving code in order to see how it all
works out.  It did well, so don't spam the log anymore.

The "register access failure" situation still needs an actual handler.
But at this point it makes less sense to ask folks to send mails about
it.  We now have a pretty good picture of what controllers emit this and
when:

Texas Instruments PCIxx21 FireWire + CardBus + flash memory card
controller:
https://bugzilla.redhat.com/show_bug.cgi?id=608544

O2 Micro FireWire + flash memory card controller:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/801719
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/881688
http://marc.info/?l=linux1394-devel&m=132309283531423
http://marc.info/?l=linux1394-devel&m=132368567907469
http://marc.info/?l=linux1394-devel&m=132516165727468
http://marc.info/?l=linux1394-devel&m=133006486927699

Pinnacle Movieboard:
commit 7f7e3711


http://marc.info/?l=linux1394-devel&m=130714243325962

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