remoteproc: Refactor function rproc_cdev_release()

Refactor function rproc_cdev_release() to take into account the
current state of the remote processor when choosing the state to
transition to.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210312162453.1234145-18-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>