[SCSI] atp870u: fix memory addressing bug

From: Alan Cox <alan@redhat.com>

The virt_to_bus() wasn't correctly taken out of this driver.  It needs
to be able to track both physical and virtual addresses for its prd table.
Update the driver to do this with separate tracking entries.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>