msm: mhi_dev: Wait for pending writes to complete

Wait for pending (async) writes to complete before closing a channel.
If channel is closed, do not call client callback or send completion events
to host.

Change-Id: Iddb8320d56a6f3f1476615c097c610c883b54915
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>