hw/pci-bridge: Use the QOM BUS() macro to access BusState.qbus

Rather than looking inside the definition of a BusState with "s->bus.qbus",
use the QOM prefered style: "BUS(&s->bus)".

This patch was generated using the following Coccinelle script:

    // Use BUS() macros to access BusState.qbus
    @use_bus_macro_to_access_qbus@
    expression obj;
    identifier bus;
    @@
    -&obj->bus.qbus
    +BUS(&obj->bus)

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20190528164020.32250-4-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>