cciss: fix scatter gather cleanup problems

On driver unload, only free up the extra scatter gather data if they were
allocated in the first place (the controller supports it) and don't forget
to free up the sg_cmd_list array of pointers.

Signed-off-by: Don Brace <brace@beardog.cce.hp.com>
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>