rpmsg: pass right device paramter to dma_free_coherent function

dma_alloc/free_coherent APIs requires the platform specific remoteproc
device as the device parameter. We are passing vdev->dev.parent to the
dma_free_coherent function which is wrong, it has to be
vdev->dev.parent->parent instead as in the dma_alloc_coherent function.

Signed-off-by: Fernando Guzman Lugo <fernando.lugo@ti.com>