rpmsg: glink: fix destroy channel endpoint logic

When rpmsg client driver destroys last channel endpoint, remove rpmsg
device is triggered. In both cases (destroy endpoint and remove device)
a glink close command sent to the remote peer.

This change, when for removing rpmsg device endpoint already destroyed
will avoid sending second glink close command.

Change-Id: I0135e2626f4ede7a380308944f7c273d18e223bf
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>