rpmsg: Avoid double-free in mtk_rpmsg_register_device()

[Markus Elfring]

>> Can another imperative wording become helpful for the change description?
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=9c7d619be5a002ea29c172df5e3c1227c22cbb41#n151
>
> Looking at your posting history, I'll leave it up to the maintainer.

Would you like to follow the Linux development process documentation here
(for the subsequent patch version)?

Regards,
Markus