hw/usb-storage: Use the QOM DEVICE() macro to access DeviceState.qdev

Rather than looking inside the definition of a DeviceState with
"s->qdev", use the QOM prefered style: "DEVICE(s)".

This patch was generated using the following Coccinelle script:

    // Use DEVICE() macros to access DeviceState.qdev
    @use_device_macro_to_access_qdev@
    expression obj;
    identifier dev;
    @@
    -&obj->dev.qdev
    +DEVICE(obj)
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NGerd Hoffmann <kraxel@redhat.com>
Message-Id: <20190528164020.32250-9-philmd@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

hw/usb/dev-storage.c

@@ -616,7 +616,7 @@ static void usb_msd_storage_realize(USBDevice *dev, Error **errp)
      * The hack is probably a bad idea.
      */
     blk_ref(blk);
-    blk_detach_dev(blk, &s->dev.qdev);
+    blk_detach_dev(blk, DEVICE(s));
     s->conf.blk = NULL;
 
     usb_desc_create_serial(dev);