cciss: clean up code in cciss_shutdown

cciss: clean up code in cciss_shutdown.  Send the flush cache
command down with interrupts still enabled, and do not do DMA
from the stack.

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