[SCSI] sr: fix error handling in module_init

Sweep registered blkdev when scsi_register_driver has failed.

Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>