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>

common/splash_source.c

@@ -146,7 +146,11 @@ static int splash_init_usb(void)
 	if (err)
 		return err;
 
-	return usb_stor_scan(1) < 0 ? -ENODEV : 0;
+#ifndef CONFIG_DM_USB
+	err = usb_stor_scan(1) < 0 ? -ENODEV : 0;
+#endif
+
+	return err;
 }
 #else
 static inline int splash_init_usb(void)