rpmsg: virtio: Remove unused including <linux/of_device.h>

Remove including <linux/of_device.h> that don't need it.

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210811123125.143-1-caihuoqing@baidu.com