rpmsg: glink: Remove the rpmsg dev in close_ack

Un-register and register of rpmsg driver is sending invalid open_ack
on closed channel.

To avoid sending invalid open_ack case unregister the rpmsg device
after receiving the local_close_ack from remote side.

Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
[bjorn: s/strlcpy/strscpy/]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/1596086296-28529-5-git-send-email-deesin@codeaurora.org