ATAPI multi-lun support has been broken for a long time.

It used to be that "(drive->id->last_id & 0x7) + 1" was used as
shost->max_lun and the "hdXlun=" kernel parameter could be used to
override this value.

However it was far from optimal:
- people played with "hdXlun=" and then complained about multiple instances
  of the same device (most ATAPI drives respond to each LUN)
- probably some devices return 7 not 0 in id->last_id (=> 7 x same device)

This patch from Willem Riede <wrlk@riede.org> fixes it w/o need
for "hdXlun=" option.  It was tested by Willem on ATAPI PD/CD drive.