[SCSI] sg: set class_data after success

If cdev_add fails in sg_add, sg_remove crashes since class_data is
bogus.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>