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.

CRs-Fixed: 2275152
Change-Id: I5255c8bb55cb3bc35d9813a3a349d8af391d1862
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>