[SCSI] bsg: fix bsg_unregister_queue

scsi_sysfs_add_sdev ignores the bsg_register_queue failure, so
bsg_unregister_queue must check whether the queue has a bsg device.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>