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: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20190528164020.32250-9-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>