virtio-mmio: Use to_virtio_mmio_device() to simply code

[ Upstream commit da98b54d02981de5b07d8044b2a632bf6ba3ac45 ]

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>
Stable-dep-of: 55c91fedd03d ("virtio-mmio: don't break lifecycle of vm_dev")
Signed-off-by: Sasha Levin <sashal@kernel.org>