firewire: sbp2: give correct DMA device to scsi framework

The sbp2 driver does DMA not on the unit but on the card device.

The driver worked even with the wrong device because at the moment, it
happens to reimplement the DMA functions of the SCSI framework.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>