[SCSI] pm80xx: 4G boundary fix.

Firmware is having an issue.  When a single IO request crosses
4G boundary, system will crash.  To avoid the issue single sg is
converted into extended sg.

Signed-off-by:  <Anandkumar.Santhanam@pmcs.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>