coresight: perf: Fix NULL point dereference issue

In the etm_setup_aux(), if no sink is found, then the path is NULL.
If etm_free_aux() is called directly, it will cause null-pointer
dereference. this change clear cpu mask to avoid this issue.

Change-Id: I392c04cb4ac75755a04a0e100450aeb2892e3812
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>