scsi: sr: Fix sr_probe() missing deallocate of device minor

If the cdrom fails to be registered then the device minor should be
deallocated.

Link: https://lore.kernel.org/r/072dac4b-8402-4de8-36bd-47e7588969cd@0882a8b5-c6c3-11e9-b005-00805fc181fe


Signed-off-by: Simon Arlott <simon@octiron.net>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>