diag: Add mutex protection while closing diag channels

Add mutex protection while opening and closing diag channels to
fix the race conditions.

Change-Id: I26bc5e4061bd86f112993ac16789a95e6d6e01c8
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>