rpmsg: Avoid double-free in mtk_rpmsg_register_device

If rpmsg_register_device fails, it will call
mtk_rpmsg_release_device which already frees mdev.

Fixes: 70179969 ("rpmsg: add rpmsg support for mt8183 SCP.")
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200903080547.v3.1.I56cf27cd59f4013bd074dc622c8b8248b034a4cc@changeid


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