diag: Avoid race while closing and opening mhi channels

Add protection while closing mhi channels to ensure that close
has finished completely before calling open.

Change-Id: Ib112f5b7d38aee154f2938dab8b314eb1c3b7e7a
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>