libata_scsi: Fix ATAPI transfer lengths

Some controller variants snoop the ATAPI length value for Packet
transfers to do state machine and FIFO management. Thus we want to
set it properly, even for cases where it is otherwise meaningless.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>