[SCSI] scsi_transport_spi: fix oops in revalidate

The class_device->device conversion is causing an oops in revalidate
because it's assuming that the device_for_each_child iterator will only
return struct scsi_device children.  The conversion made all former
class_devices children of the device as well, so this assumption is
broken.  Fix it.

Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>