[SCSI] wd7000: proper fix for boards without sg support

  - code used to set sg_tablesize to zero for board revision
    less than 6. This is no longer supported, therefore I
    use sg_tablesize=1 and open code the sg handling for that case.
  - Get rid of use of SG_NONE which will be removed soon.

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