[SCSI] hpsa: mark last scatter gather element as the last

This is normally optional, but for SSD Smart Path support (in
subsequent patches) it is required.

Signed-off-by: Matt Gates <matthew.gates@hp.com>
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>