dm: usb: Move storage device scanning into its own function

The usb_stor_scan() function is quite long, so split out the code that scans
each device into its own function. Also, rather than setting up the block
device list once at the start, set it up as each device is scanned. This
makes it possible to use this code from driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>