cciss: fix flush cache transfer length

We weren't filling in the transfer length of the
flush cache command (it transfers 4 bytes of zeroes).
Firmware didn't seem to be bothered by this, but it
should be fixed.

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