The scsi_level value in struct scsi_device is a bit
mysterious. Hopefully this patch makes it a bit clearer
since it is visible via sysfs.

Changes:
  - add scsi_level constants for SPC-2 and SPC-3
  - comment on relationship between scsi_level and INQUIRY response
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>