remoteproc: Fix wrong rvring index computation

BugLink: https://bugs.launchpad.net/bugs/1878098

commit 00a0eec5 upstream.

Index of rvring is computed using pointer arithmetic. However, since
rvring->rvdev->vring is the base of the vring array, computation
of rvring idx should be reversed. It previously lead to writing at negative
indices in the resource table.
Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20191004073736.8327-1-cleger@kalray.euSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Doug Anderson <dianders@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/remoteproc/remoteproc_core.c

@@ -288,7 +288,7 @@ void rproc_free_vring(struct rproc_vring *rvring)
 {
 	int size = PAGE_ALIGN(vring_size(rvring->len, rvring->align));
 	struct rproc *rproc = rvring->rvdev->rproc;
-	int idx = rvring->rvdev->vring - rvring;
+	int idx = rvring - rvring->rvdev->vring;
 	struct fw_rsc_vdev *rsc;
 
 	dma_free_coherent(rproc->dev.parent, size, rvring->va, rvring->dma);