ieee1394: sbp2: let nodemgr retry node updates during bus reset series

sbp2 was too quick to report .update() to the ieee1394 core as failed.
(Logged as "Failed to reconnect to sbp2 device!".)  The core would then
unbind sbp2 from the device.

This is not justified if the .update() failed because another bus reset
happened.  We check this and tell the ieee1394 that .update() succeeded,
and the core will call sbp2's .update() for the new bus reset as well.

This improves reconnection/re-login especially on buses with several
disks as they may issue bus resets in close succession when they come
online.

Tested by Damien Benoist.

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