rpmsg: Don't overwrite release op of rpdev

b0b03b81 ("rpmsg: Release rpmsg devices in backends") attempted to
correct the ownership of freeing rpmsg device memory. But the patch
is not complete, in that the rpmsg core will overwrite the release op as
the device is being registered.

Fixes: b0b03b81 ("rpmsg: Release rpmsg devices in backends")
Reported-by: Henri Roosen <henri.roosen@ginzinger.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>