splash: Accommodate DM_USB in splash_init_usb()

Current implementation of splash_init_usb() requires usb_stor_scan()
which doesn't exist in case of DM_USB simply because real probing
happens right in usb_init().

So disable usage of usb_stor_scan() in case of DM_USB.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Robert Winkler <robert.winkler@boundarydevices.com>
Reviewed-by: Simon Glass <sjg@chromium.org>