dm: usb: Tidy up storage code ready for driver model conversion

Adjust a few things so that the addition of driver-models support involved
adding code rather than also changing it. This makes the patches easier to
review.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Stephen Warren <swarren@nvidia.com>