scsi: core: remove unnecessary unlikely()

BUG_ON() already contains an unlikely(), there is no need for another one.

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: linux-scsi@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>