remoteproc: Fix wrong rvring index computation

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>

drivers/remoteproc/remoteproc_core.c

@@ -399,7 +399,7 @@ rproc_parse_vring(struct rproc_vdev *rvdev, struct fw_rsc_vdev *rsc, int i)
 void rproc_free_vring(struct rproc_vring *rvring)
 {
 	struct rproc *rproc = rvring->rvdev->rproc;
-	int idx = rvring->rvdev->vring - rvring;
+	int idx = rvring - rvring->rvdev->vring;
 	struct fw_rsc_vdev *rsc;
 
 	idr_remove(&rproc->notifyids, rvring->notifyid);