diag: Protect the decrement of number of diag clients

In diagchar_open() protect the decrement of number of diag clients
so that there will be no race conditions while reading the value
from other functions.

Change-Id: I0e2fb5331eec9c7bba39e7d881b69559256833a3
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>