[SCSI] scsi_transport_spi: fix for unbalanced reference counting

Check the domain validation flag on the given device before referencing
scsi_device instance, otherwise if the flag is already set we return without
decrementing the reference count.

Signed-off-by: Mike Maslenkin <mihailm@parallels.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>