scsi: Implement sg_printk()

Update the sg driver to use dev_printk() variants instead of
plain printk(); this will prefix logging messages with the
appropriate device.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Acked-by: Doug Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>