[SCSI] nsp_cs: convert to data accessors and !use_sg cleanup

  - use scsi data accessors
  - cleanup !use_sg code paths
  - TODO: use next_sg() for Jens's sglist branch. Look for 2
    places with "SCp.buffer++"

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>