scsi_debug: define pr_fmt() for consistent logging

Use pr_fmt with both module name and __func__
Also drop few bare printk leftovers

The log format should stay pretty much intact

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>