virtio-mmio: Use to_virtio_mmio_device() to simply code

The file virtio_mmio.c has defined the function to_virtio_mmio_device,
so use it instead of container_of() to simply code.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20210222055724.220-1-tangbin@cmss.chinamobile.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>