scsi_logging: return void for dev_printk() functions

dev_printk() is now a void function, so the related functions
scmd_printk() and sdev_prefix_printk() should be made void, too.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>