cciss: simplify scatter gather code

cciss: simplify scatter gather code.
Instead of allocating an array of pointers to a structure
containing an SGDescriptor structure, and two other elements
that aren't really used, just allocate SGDescriptor structs.

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