[SCSI] scsi_dh_rdac: Fix error path

If create_singlethread_workqueue() failes, rdac_init should fail too.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: "Moger, Babu" <Babu.Moger@netapp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>