[SCSI] st: clean up dev cleanup in st_probe

st_probe leaves a cdev pointer hanging around that is compared
during the error path and freed later. There's no need for the pointer
to hang around at all. So we free it immediately and simplify the error
handling.

Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>