dm: scsi: Use the uclass platform data

At present the two driver-model SCSI drivers use device platform data to
store information that relates to the uclass. It is better to use uclass
platform data in this situation. Update the code to do this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>