rpmsg: char: Fix race between the release of rpmsg_ctrldev and cdev

rpmsg: char: Fix race between the release of rpmsg_ctrldev and cdev

This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven.
This ID is fixed in Linux Kernel version v4.14.265 by commit 74d85e9fbc7022a4011102c7474a9c7aeb704a35, it was introduced in version v4.11 by commit c0cdc19f84a4712cf74888f83af286e3c2e14efd. For more details please see the references link.