Fix compilation error in cmd_usb.c

This patch fixes compilation error
cmd_usb.c: In function 'do_usb':
cmd_usb.c:552: error: void value not ignored as it ought to be

Signed-off-by: Anatolij Gustschin <agust@denx.de>

include/usb.h

@@ -195,7 +195,7 @@ void usb_event_poll(void);
 #define USB_MAX_STOR_DEV 5
 block_dev_desc_t *usb_stor_get_dev(int index);
 int usb_stor_scan(int mode);
-void usb_stor_info(void);
+int usb_stor_info(void);
 
 #endif