scsi: esp_scsi: use strong typing for the dev field

esp->dev is a void pointer that points either to a struct device, or a
struct platform_device.  As we can easily get from the device to the
platform_device if needed change it to always point to a struct device
and properly type the pointer to avoid errors.

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>