coresight: Forbid enabling sink while reading sink

Forbid enabling sink in the process of reading sink. Otherwise,
sink flag "activated" will be set to true, and the source has a
chance to be added to "cs_active_paths" twice. Then, system will
crash if sink is continued to be read due to two same source in
"cs_active_paths".

Change-Id: I3317ea435e9d84258e3fcbf854a6078afcd4a08e
Signed-off-by: Tao,Zhang <taozha@codeaurora.org>