ieee1394: sbp2: more concise names for types and variables

"struct scsi_id_instance_data" represents a logical unit.
Rename it to "struct sbp2_lu", and "scsi_id" to "lu".
Rename some other variables too.
Wrap almost all lines after at most 80 columns.

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