[SCSI] st: convert class code to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the scsi tape class code to use
the correct field.

Cc: Kai Mäkisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>