rpmsg: fix dma_free_coherent dev parameter

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 the generic rproc device and it is
wrong, it has to be vdev->dev.parent->parent instead, same as when we
call dma_alloc_coherent function.

Signed-off-by: Fernando Guzman Lugo <fernando.lugo@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>