scsi: wd719x: use per-command private data

Add the SCB onto the scsi command allocation and use dma streaming mappings
for it only when in use.  This avoid possibly calling dma_alloc_coherent
under a lock or even in irq context, while also making the code simpler.

Thanks to Ondrej Zary for testing and various bug fixes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>