[SCSI] st: fix up after class_device removal

There's a change in the SCSI tree that adds another class_device, so change
it to an ordinary device

[jejb: this one got rebased until it's basically cosmetic only]

Cc: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>