scsi: esp_scsi: remove union in esp_cmd_priv

The dma_addr_t member is unused ever since we switched the SCSI
layer to send down single-segement command using a scatterlist
as well many years ago.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>