scsi: use 'scsi_device_from_queue()' for scsi_dh

The device handler needs to check if a given queue belongs to a scsi
device; only then does it make sense to attach a device handler.

[mkp: dropped flags]

Cc: <stable@vger.kernel.org>
Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>