[SCSI] gdth: !use_sg cleanup and use of scsi accessors

gdth_execute() will issue an internal, none scsi-standard commands
onto __gdth_queuecommand(). Since it is not recommended to set
struct scsi_cmnd IO members in llds, gdth now uses internal IO
members for IO. In the case of gdth_execute() these members will be
set properly. In case the command was issued from scsi-ml
(by gdth_queuecommand) they will be set from scsi IO accessors.

  * define gdth IO accessors and use them throughout the driver.
  * use an sg-of-one in gdth_execute() and fix gdth_special_cmd()
    accordingly.
  * Clean the not use_sg code path and company

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