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.eu


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>