cciss: Dynamically allocate struct device for each logical drive as needed.

Dynamically allocate struct device for each logical drive as needed
instead of allocating the maximum we would ever need at driver init time.

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