diag: Don't write to glink channel while closing channel

Don't write any data to glink channel when diag receives close
notification on diag glink channel.Avoid race condition while
closing the channel and writing to the channel.

Change-Id: Ibd9ca9fec9051bce596477977a05786fb8ec6f97
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>